Fellow wonderbreads, protip: NEVER NEVER ask if you can touch their hair. You can't. AND FOR GOD'S SAKE DON'T JUST DO IT.
Just, just no
But that's just generalisation, too. When I first met my black wife, she would not take her wig off, like, ever. She thought her natural hair was disgusting, and everyone would hate her if they saw it. At that point in time, I'm pretty sure she would have freaked out if someone touched her. (mostly because they took her wig off).
IIRC the transistion began when she watched a German video where a white blonde used curlers to get very finly curled hair. She just couldn't understand why anyone would do that to themself - she had such nice, straight hair, why ruin it? How could curly hair be considered beautiful?

It did not fit her worldview...
When she came to live with me in Austria, I told her she does not need a wig. Having people ask to touch it and admire it did a lot, as it helped her understand that, quite genuinly, her hair is beautiful and nobody hated it. People even told her they would like to have it. It's still a pain to treat it, but by now she only wears wigs as a fashion item, not to hide her hair. And she never complained about anyone wanting to touch her natural hair here.
So my take on it: Appearently young black girls get peer-pressured into thinking their natural, curly hair is ugly, and from there, they either hide it or use products on it. They are ashamed of their hair in it's natural state, and to me, that explains why they would not want anyone to touch it. I might be completely wrong - but as a guy with long hair who had people ask to touch my hair, I wouldn't know what's wrong with that. (Then again, "Your tits look amazing, can I touch them?" is surprisingly similar...)