he restaurant is not actually asking customers whether they voted for President-elect Trump, and said even if they see a customer with a Trump shirt, “we don’t put anything different [in] your food.”
“Robert just wants to express how much he doesn’t like Trump”
Well, he
says he won't put anything different in the food. I guess the good news is that he's upfront with the sign. If you really like his food, go eat it (but still don't wear a "Trump" shirt) or else know to cross the place off your list. The ones I would worry about are those that feel as he does but don't have signs. Walk into their restaurants with a Trump shirt, and you may or may not end up with trouser food.
As a private business, he can do what he wants, though the "won't serve you, you nazi!" is a bit off-putting. I prefer the way they do it at a place I go to when in Mammoth Lakes, The Outlaw Saloon. They have the below sign, which states their political view, but doesn't do the "won't serve you" thing. Actually their sign is more of a gutsy move than the Hawaii guy's sign, since the Hawaii guy is safe in his majority. Mammoth is a liberal conclave in otherwise conservative Mono County, so the saloon has a lot more business to potentially lose (though they're always pretty packed).