Had he done what you say the Media would have turned it into a “Trump is a racist story” and lied about what really happened to push that meme for the next two weeks. As it is it’s a non story. It won’t cost him a single black vote and may actually gain him some. Trump did the right thing.
It’s a crack in his armor. He’s never let himself be manipulated into someone else’s agenda before. He can handle himself; I doubt this will go too far.
But if you’ve made your bones as a hard-liner, you don’t go squishy in the back stretch.