The "promise" to the Creek isn't worth the paper it's written on because they don't have enough military might to enforce the terms of the agreement. The US operates with them as it does with every one else, on the principle of "might makes right."
This is patently false, because the Supreme Court has ruled that the promise is enforceable.
> The US operates with them as it does with every one else, on the principle of "might makes right."
This is also completely false. Does the military answer to the President of the US because "might makes right"? Can Trump overpower any enlisted man in the military with a gun? The military obeys the Constitution because they swore an oath to do so, not because the generals have literal guns to their heads.
Culture and rule of law matter. The statement "might makes right" is a gross oversimplification that doesn't reflect our reality at all.
Sure, Congressional legislation could take away that land. That would be a lousy thing for them to do. Unfortunately since I doubt Native American groups are a big enough voting bloc, the only thing that would stop Congress from doing that would be enough negative public opinion from people who are not Native American. Hopefully there are enough people of that sort.