Well sure, they drive controversy, which means headlines, and Trump is first and foremost a showman. No sane politician is going to intentionally drive away potential voters, and as for the racists themselves, Trump is still the closest thing to an acceptable candidate they've had since the 60's, so they're going to idolize him as their Chosen One. He's going to milk the controversy for all it's worth, string them along as long as possible. For that matter, it could easily have been a 'mistake' as far as intended to stir up controversy like every other aspect of the Trump campaign. I'm just quite certain it wasn't intended to be a genuine candidate that Trump then backpedaled on, because the one thing he's been very consistent on is that he doesn't care what anyone else thinks. Even the David Duke thing he just waffled about and acted ignorant of for publicity.
Ultimately, if I had to make a unsupported assessment of Trump's actual attitude, I'd say he is anti-foreigner, rather than simply racist. He's made comments against Hispanics/Mexicans, against Muslims, against most people who aren't American - he wants protectionism re-instated. But he doesn't think of black people as 'foreign', so they don't register on his radar at all (ignoring the question of African immigrants, but no one asks about that for obvious reasons). The racists see him railing against Hispanics/Arabs and assume, like I think you might be, if you're racist against one group you must be racist against all of them.