If USA maintains nukes in Turkey that does not mean they have enough troops in Turkey to protect the base against the entire Turkish military, should they decide to steamroll it. They rely on nuclear codes more than they rely on tanks to stop nukes falling into the wrong hands.
They don't actually have to. The 5th and 6th fleet coupled with the various large air bases in the vicinity would have little trouble it pancaking the entire site before the Turks managed to make off with anything if it really came down to it. It's tanks all the way down.
Turkey allows those troops to be there. Ukraine did not want Russian troops on their soil. Forcibly ejecting those troops would have likely triggered a confrontation with a far more competent Russian military than the one we see today.
