I suspect that Trump and the majority of Americans think thusly about foreign relations: "It is America's manifest destiny and sacred duty to control (er, "protect") the rest of the world (foreigners certainly can't be trusted to do it themselves--they might not even be able to identify the proper "enemy") but, by jingo, we expect the rest of the world to PAY for this benevolent management we're providing. Like Mexico and that wall we're gonna build down there for their own good. Enough unfair exploitation of American taxpayers! No more free military occupation! Pay up, you moochers!"
Throughout history, this has generally been the prevailing empire's argument--and justification--for demanding financial "compensation" from its subject peoples. The British did this in 18th century North America in an effort to exact some payment for the "protection" they had provided the cheapskate colonies in the French and Indian (Seven Years') War. The demand was probably legitimate: the redcoats HAD after all kept out the poxy French--and the war had been costly.
Nonetheless, the methods employed (taxes levied from London with no colonial input) were ill-conceived, ill-executed, and shit! they just pissed everybody off so bad! Consequently, in that instance at least, the subject people not only refused to pay--but they tossed the arrogant Imperials out on their rosy, red arses. Achtung! therefore, Mr. Trump, lest the NATO leaders grow so angered by YOUR methods that they decide to call your bluff and, following the example set back in 18th century Boston, dump you (and America) unceremoniously into the sea--like a fat bag of Oolong.
That would probably get you very bad ratings.