The Kurds lost thousands of men in defeating ISIS. The US lost a dozen or so. The US military is heartsick that Trump is forcing our soldiers to abandon their comrades in arms, a betrayal that will haunt the US when they next seek allies to fight our fights.
Now you bring up the point: the US is using the Kurds to defeat ISIS that's all, now that ISIS is defeated; and they don't want to fight against Iran, there is no more mutual benefits, so the US cuts the cord. No loyalty, no free lunch in this world, trust me ! every country, every nation is just a small pawn in a chess board manipulated by the big guys !
By the way, who created this mess in Syria? if not saved by Russia, Bashar Al-Assad will end up like Kadhafi and Sadam by now!
Ask the Syrian people, they will tell you: better govern by a Dictator and have a job, peace, a house, and family; than fleeing Syria and live in a refugee camp with nothing now.