By the US, you mean Rooservelt, because the US actually did help a few of its western allies (i.e France) try to keep their colonial domains after 1945. Especially with the start of the cold war. But also helped the colonized/ex-colonized (i.e Suez, Indonesia) after the war.

But Roosevelt's basic view was that European imperialism was much responsible for creating WWI and WWII in the first place, plus, he, at least by some of his comments, genuinely seem to think that colonialism was wrong and sympathized with colonized people's nationalist aspirations.

source: Warlords: An Extraordinary Re-creation of World War II through the Eyes and Minds of Hitler, Churchill, Roosevelt, and Stalin

The U.S supported and or left France and the UK to deal with their colonies as they pleased. The French in a time where they felt they needed to reestablish their military mark in the world tried to reclaim their territories. France succeeded in keeping some small territories, but lost both Indochina and Algeria due to massive independence wars that had a large part in the collapse of the Fourth Republic. As the Cold War ramped up, following Roosevelt's or Wilsonian ideals took second place behind fighting Communism. And so the U.S supported France's failed efforts in Vietnam. As for Algeria and the Dutch in Indonesia I am not knowledgeable enough to comment on any U.S intervention.

As for the U.K, the empire was financially crippled and cutting their losses and the potential for vast resistance movements was far more important than any U.S concerns.