Because it's saying you only care about a small percentage of people. To quote the POTUS, "we all have the same colour blood". Why then should Trump go to a huge effort (which typically means paying the CEOs handouts to move factories) to give an American a job rather than someone else?
Haha yes. God was always a fan of exclusionary policies that benefit only certain people on this earth. If I remember correctly the bible states:
"Do unto others as you would have them do unto you, so long as they are from America" :)