There are some international treaties US is part of that mandate certain behavior from US to non-US citizens in the US. It depends on the particular citizenship. There's requirement to allow access to consulate, for some countries of origin there's requirement to notify consulate if a citizen of that country was detained, etc.

US gov. can violate these at their own peril. It's all based on reciprocity, assuming US citizens would also get worse treatment elsewhere.

At least not since Obama missiled a US citizen and his teenage son w/out charge.

Depends on your definition of trial. Non US citizens held in areas that are de facto controlled by the US have basic habeas rights:

https://en.m.wikipedia.org/wiki/Boumediene_v._Bush

That'd not hold for areas of active hostilities however.

IANAL.

Noncitizens in the US also have the right to a trial. Actually, the US has something of a tradition of declaring that noncitizens outside the US have somehow come under US jurisdiction (e.g., the people who killed Daniel Pearl).