I can understand why they may have had reservations about bombing Auschwitz or the rail lines leading to it as there were inmates there that they wanted to avoid killing and it wasn't just a death camp, but what about say Treblinka which was a pure killing factory? Were they unaware of what was happening there?
The Operation Reinhard camps had been dismantled by the time Western Allied bombers (who I presume you are asking about) were able to penetrate that far into enemy territory, i.e. Eastern Poland, very close to the border with the Soviet Union.
The Soviets, on the other hand, had suffered heavy losses to their airforce during Operation Barbarossa (the German invasion of the Soviet Union in the summer of 1941) and subsequently had to concentrate their remaining (and newly built) planes on the fight with the enemy for their own territory.
In addition, these camps were only operational for a relatively short time, less than two years between Spring of 1942 and Summer/Autumn of 1943. Some reports of their purpose and methods had been conveyed by the Polish resistance to the Polish government abroad and from them to the Western Allies but in addition to the fact that they could hardly be reached by Western forces, there were other factors that would have prevented military action, mainly disbelief of these "atrocity stories", and the fact that the shutting down of them served no strategic purpose whatsoever. The Allies were concentrating on winning the war, not necessarily on saving specific civilian populations.
The Holocaust has come to symbolise the evil of the Second World War in our days, but it was really just another, albeit tragic, story of wartime suffering at the time. The full scale of the horror was not recognised until later.