Those troops are there by invitation.
Like the Soviet troops were "by the invitation" of the SED in the DDR.
That said, I do agree it's time Germany and our other "allies" grow up and defend their own nations.
"Grow up" is a stupid expression.
Nato was created to "to keep the Russians out, the Americans in, and the Germans down,"
(Hastings Ismay, first Secretary General of NATO from 1952 to 1957)
Hastings Ismay, 1st Baron Ismay - Wikipedia
After the collapse of the East Block and the Warsaw Treaty, there was no justification for NATO and for keeping Germany down any more.
We need to withdraw from Germany at a rate that allows Germany to increase the size of their military to balance the withdrawal.
The new confrontation with Russia was created artificially, to justify the going on with "keeping Germany down".
The real threat comes not from Russia, but from the Islamic World, and to prevent this threat you have to protect your boarders, not to increase your military.