The US acted with a lot of contempt for "commies" as they called them. The reason they didn't attack was not because of magnanimity but the very fact that world had exhausted itself with the bloodiest war in mankind over the last 6 years. By 1949, the cat was out of the bag with the RDS-1 with USSR. Until 1952, US didn't have a viable means to fly several heavy thermonuclear devices to a heavily fortified Russia & drop it (The B-52 Flying fortress was the first viable delivery vehicle which could fly high & heavy)

And they did try, if you remember Korean War happened in 1950, which was a push to terminate communism in Asia. That failed. It was not because of a lack of bombs - but winning a war takes boots on the ground eventually. And US post-war, as mentioned, had just started ramping on its military might again after the heavy material losses. WW2 was not a PlayStation game fought with cheats. US lost lots of machinery & money in the process- and desperately needed to recuperate. Driving a weathered military to fight the brutal Red Army was unthinkable (Fun fact: Japan did not surrender because of the bombs, but an impending Russian invasion very soon. They were preparing to keep fighting on. Russian brutality was a precedent that the Imperial mandarins were not prepared to face & Sakhalin was already seeing Red Army buildup. They would have exacted their revenge for their 1905 defeat with heavy costs)

Whats the point of dropping a few bombs? US didn't have enough nuclear materials post-war immediately & USSR was a vast country sparsely populated except Moscow & Kyiv. If in doubt, just look at the map of USSR & how spread out the military bases were.

Summarily, it was not magnanimity but a lack of provocation, opportunity & resources to carry out a total annihilation of its adversaries.

edit: more context