If one day germany, japan or south korea were to decide to have referendum to remove US troops and get majority vote, I am sure US would remove their troops. So in short: no, US is not holding a gun to them because the other countries are certainly enjoying US nuclear and military umbrella protecting their territory.
Yes, they 100% have a choice. Why do you think otherwise? They're US allies and they benefit from the situation.
Another poster pointed out that they benefit from this arrangement by not having to spend so much on their domestic militaries to protect themselves.
Another point: in the case of South Korea and Germany, the US military helps protect those countries from the real threats of invasion. South Korea is still technically at war. For 50 years US soldiers eyed Soviet soldiers across the inner German border, and closely worked with West Germany and NATO to be prepare if those soldiers ever crossed over.
You have a very odd, binary perspective on international relations that seems to preclude win-win military cooperation.