Why is an "America First" position something to be excoriated?  
I've seen a number of economists rip it as bad policy to the extent it involves tariffs, trade wars, etc.  That I fully understand.  
But I've also seen a good bit of non-economist criticism of an America First policy, on the basis that it's somehow wrong (unethical, immoral... it's never fully fleshed out) to put US interests first.  But... isn't that exactly what we do all the time?  Isn't that kind of an essential behavior of a state -- that it act first and foremost in the interests of its citizens initially, all others secondarily?  Hasn't our national policy, and the national policies of every other state, been to act in its own interests?  
If this seems to borrow some logic from Louis CK's 
"Of course, but maybe..." routine, it's intentional.