It's not so much about 'why the hell not' as about 'why the hell yes'. The consequences may not be all that dire in the end, but it's hard to see much in the way of objective benefit or gain for the US that outweighs the bad. Gain for Trump, certainly - he scores brownie points with evangelicals and with his supporters who get to praise him for actually keeping promises (as long as they don't involve anything that could directly or indirectly touch his pocketbook), and the more he's vilified abroad the more his fans at home love it. But for the US as a whole?

It's one thing to say the US should be less concerned about how other nations view it when the US openly pursues its purely national interests rather than international ones. But hard to see how this kind of move is even a national interest at all.

The only possible advantage I see, and that's really kind of a stretch, is if you want to claim that Trump acting like Erdogan and telling the world 'America's going to start basing its foreign policy on what plays well with the home voters, not on anything else' will ultimately force the other players on the world stage to step up and play larger roles. But while such shifts would be useful to the US in some regards, surely they would also be damaging in others.