The short answer is that that to prevent it, they would have to shoot down incoming and outgoing aircraft, and that would have been an act of war. Stalin may have been a paranoid tyrant entering the final years of his grotesque life, but he wasn't without intelligence and insight.

For context you might want to read this post I did on the USSR and the Cold War's origins, this post on the Truman Doctrine, and perhaps even this post on the Marshall Plan.

OK, that's all very glib (but I believe correct), so here's the longer version.

It's important to set the context for what happens in Berlin in 1948-49. As you'll know, Germany was divided into Soviet-controlled and American/British/French-controlled zones at the end of WWII. Germany was a core concern of the Cold War’s first period. Both the USA and USSR want to avoid resurgent German militarism. Stalin believed that all of Germany will eventually be unified under communism.

It is political and economic changes in Germany that provoke the Cold War’s first major crisis. Germany had undergone a financial crisis after the war, much of which stemmed from the agreement not to allow trade between the four zones and the dismantlement of assets within the zones of control. In January 1947 the US and UK controlled zones were economically merged, to be joined by the French zone at the start of June 1948. This unification - and the introduction of a new currency (the Deutschmark) for the unified Wester zones on June 18, 1948 - hit the Soviet zone of control hard. It was much more dependent on economic links to the rest of Germany and had been hit by US embargoes imposed in early 1948.

Now, Stalin had a desire to prevent the rise of a re-armed, independent West German republic allied to the USA. But many reasons for his precise behaviour about and around Berlin in this period are still opaque. Stalin decided on the strangulation of Berlin on 19 June 1948 and started to implement the so-called Berlin Blockade on June 24. And as we know, the 'western' response was the massive Berlin Airlift.

So, your core question: why do the Soviets not simply shoot down the freighters? At the most basic level, that would have been an act of war and would likely - although we can never be sure - have precipitated outright conflict between the USSR and the USA/UK/France, etc. On another level, Stalin, Molotov, and other key figures in the Soviet leadership did not believe that the airlift could be sustained for any significant period and they would thus be ultimately successful. On yet another level, the Truman administration engaged in a form of 'atomic' diplomacy by transferring atomic-capable 'Silverplate' B-29 bombers to the UK. It was made clear that these aircraft were 'atomic capable' but never outright stated if they had their atomic payloads with them (they didn't). Finally, Moscow was waiting on the results of the 1948 US presidential election. Like many observers in the US, they were convinced that Thomas Dewey was going to win and this - through the influence of key Congressional figures like Taft - would result in a 'return to isolationism' and a US withdrawal from European affairs. So it is clear that the Soviet reason around Berlin was informed by a very complex calculus that drew in a whole range of different geopolitical, political, economic, logistical, and ideological factors.

As a bit of an interesting sidenote, would Truman have used the bomb to 'free' Berlin? In his private diary entry from September 13, 1948, when the Blockade was ongoing, he confided that he had the terrible feeling that, after meeting with a cavalcade of senior advisers, the world was very close to war. In a poignant three word sentence, he declared "I hope not." In conversation with David Lilienthal - then head of the Atomic Energy Commission - in February of 1949 he commented candidly "Dave, we will never use it [the A-bomb] again if we can possibly help it. But I know the Russians would use it on us if they had it."

So, there you go. There's a lot more detail to it than that, but that hopefully sketches out the main reasons for why the first Berlin crisis din't become a shooting war.

Malcolm

Sources

Truman quotes are taken from:

Ferrell, Robert H. (ed.), Off the Record: The Private Papers of Harry S. Truman (New York, 1980), 148–149.

Lilienthal, David E., The Journals of David E. Lilienthal, Vol.2: The Atomic Energy Years, 1945–1950, (New York, 1964), 474.

Secondary Materials

Craig, Campbell, and Sergey Radchenko, The Atomic Bomb and the Origins of the Cold War (New Haven, CT: Yale University Press, 2008)

Haslam, Jonathan, Russia's Cold War: From the October Revolution to the Fall of the Wall (New Haven: Yale University Press, 2011)

Mastny, Vojtech, The Cold War and Soviet Insecurity: The Stalin Years (New York/Oxford: Oxford University Press, 1996)

Pennacchio, Charles F., ‘The East German Communists and the Origins of the Berlin Blockade Crisis’, East European Quarterly 29:3 (1995), 294–314

Stivers, William, ‘The Incomplete Blockade: Soviet Zone Supply of West Berlin, 1948–49’, Diplomatic History 21:4 (1997), 569–602.

Trachtenberg, Mark, A Constructed Peace: The Making of the European Settlement 1945–1963 (Princeton: Princeton University Press, 1999)

Just to slightly develop a couple of points, the routes used to fly in to Berlin had been formally defined in late 1945 as Air Corridors - see Arrangements regarding a system of air corridors for flights between Berlin and the western zones of occupation in Germany from the Office of the Historian for notes and minutes of the meetings where they were agreed. There were no such formal agreements for road or rail access; when planning had started in 1943 for the occupation of Germany, eventually resulting in the four zones with joint occupation of Berlin, the assumption was that wartime co-operation between the Allies would continue. As Berlin was entirely in the Soviet zone it was implicit that road and/or rail access would be required to supply the western sectors, and though the subject had been raised nothing was formally signed before relations started to deteriorate. As such, technically the Soviet Union was not violating any agreement by not permitting land access to Berlin in 1948. In practice either side could have forced the issue militarily, as /u/DrMalcolmCraig says it was a complex time with many factors on both sides influencing decisions, not least doubts over the feasibility of supplying Berlin entirely by air - it was far from "simply" flying supplies in.

Over two million people lived in West Berlin, requiring some 1,500 to 2,000 tons of food every single day, plus over 3,000 tons of fuel for heating and power generation. The C-47 / Dakota, the standard Allied transport aircraft of the Second World War, could carry about 2.5 tons. Looking at the war for precedents that may have influenced the Soviet view, during the siege of Leningrad Soviet air forces had managed to deliver around 80 tons of supplies per day in 1941; German air force attempts to supply the surrounded Sixth Army at Stalingrad in 1943 never reached 150 tons per day.

Granted the circumstances of the Berlin blockade were very different, with the RAF and USAF having more and larger transport aircraft and not facing active air defences; they also had perhaps a better wartime precedent in the airlift of supplies from India to China over "The Hump" of the Himalayan mountains. At its peak this effort transported 71,000 tons in July 1945, and even managed to fly over 5,000 tons in a single day on August 1st 1945. That required over 1,000 sorties from more than 600 aircraft, though, and in 1948 USAFE (United States Air Forces in Europe) had two troop carrier groups with about 70 operational C-47s between them.

The airlift began, using the Air Corridors, as a means of supplying the military garrisons in Berlin and to at least buy some time for negotiations by demonstrating resolve. Ernest Bevin, the British Foreign Secretary, "did not imagine that it would be possible to feed two million Germans" but approved an airlift to boost morale and show strength and determination. General Clay, the American military governor, was similarly sceptical, initially considering it "absolutely impossible" to fully supply the city by air alone, but pushed for maximum effort to sustain the situation until negotiations resolved the crisis.

As it turned out the air forces, with huge effort, were able to fly in over 5,000 tons per day by early 1949 and around 8,000 per day by the time the blockade finished, but that was by no means a given when the crisis started and not fully anticipated by either side.