The US values many things in its foreign policy, and some of those things are spreading democracy and self-determination. That doesn’t mean the US values them to the exclusion of other interests like trade partnerships, energy security, regional stability, and military alliances.

Countries in the US sphere of influence tend to drift toward becoming democracies, in part because of the influence of free trade. Both Taiwan and South Korea started off as dictatorships propped up by the military backing of the US to contain the spread of communism, and eventually transitioned to democratic government.

As for Tibet, if it were on an island and could have been defended by the US Navy, I’m sure it would still be independent. Even if it were merely coastal it might still be independent. But it’s landlocked and mountainous, and hard to defend from the other side of the world. The invasion was also rapid, and happened at the same time the international community was responding to the invasion of South Korea.