Why would America care about a Communist takeover compared to the Nazis?
One isen't nessicerily better than the other, but if you're the one paying the price to instigate the change (IE backing Britain in fighting a long, bloody, and expensive "hot war" and invasion of the Continent, as suggested), you expect to be the ones getting the benefit rather than allowing the third side to reap all the profits. It'd be the same reason they woulden't attempt a "Blue Dawn" scenario to invade the Soviets, only to open up the route for Barbarossa Part II: Electric Boogaloo and allowing their other rival to swallow up the profitable parts of the USSR at essentially no cost.
Sure, America is going to back Britain, but that dosen't mean they'll do anything and everything the most militant voices in London desire.