I am partially concerned about the sheer logistics of the matter--while two decades certainly is a lot of time to develop military hardware and the bombing campaign on Laos lasted longer, I can't quite grasp how an officially unacknowledged side campaign managed to produce more explosives than the entire military might of the industrialized world.
Secondly, while I understand that there was a high degree of operational inertia and that the poor and rural Laotian population was easy to write off, did anyone in the US military argue that things were going a bit overboard?
It's categorically false to claim that the Laotian bombing exceeded all bombing in Europe in WWII. A quick google search of articles that make the claim shows they assert that the United States dropped either "more than 2 million tons", or "about 2.5 millions tons" of bombs on Laos from 1964-1973. This is significantly less than the combined tonnage of bombs dropped by British and American strategic bombers (2,770,000 tons approximately). Once German and Soviet strategic bombing, as well as tactical bombing by medium bombers, dive bombers, and fighter-bombers, are taken into account (to say nothing of artillery) there's no way it comes even close.
But, if you're asking why so many (2 million tons is still a LOT) could have been dropped on one rural country in Vietnam Southeast Asia, a big factor in that is that bombers had become so much more capable in twenty years. While a B-17 had a maximum long-range bomb load of 4,500 pounds and a maximum short-range load of 8,000 pounds, modified B-52Ds could carry 60,000 pounds of bombs. During 12 days in December 1972, 729 sorties of B-52s dropped slightly more than 15,200 tons of bombs on North Vietnam. A similar tonnage could only have been achieved by about 6,800 B-17s. Add to this that there was virtually no effective air defense in the areas of Laos being bombed, and it's not hard to see how a relative few of these bombers managed to drop a tremendous amount of munitions.
Strategic Bombing Survey
To address the second part of your question, the bombing was massed in the remote, sparsely populated (Laos had a very small population, around 2.5 million) area at the border with Vietnam. This was the 'Ho Chi Minh trail'. Keep in mind that the Democratic Republic of Vietnam did not respect borders either. It wasn't a sideshow for them. It was a vital component of their war strategy, both directing the Pathet Lao campaign in Laos and using Lao territory to send materials into southern Vietnam and Cambodia. From the US Air Force perspective, they viewed it as bombing desolate jungles filled with North Vietnamese military trucks hauling men and supplies. The bombing was also targeted on vast swaths of trees and mountains, not populated urban centers or military installations as in Japan or Europe. That hopefully explains (besides Rittermeister's points on tonnage increasing) why more tonnage was used, why it was tolerated/questioned, and also why it was not effective.