I'm a redhead. In 7th grade it was down to my waist. When I started at a new school, several of the girls at the bus stop asked to touch my hair the first day, and one reached out and did, then asked if she could braid it. I felt weird about it, but I handed her the brush from my purse and let her, because I didn't want to rock the boat. I was the new girl and I wanted to make friends, so I was happy they were being friendly to me (and I was far more used to getting teased about my hair, being a ginger makes you stick out). And yes, they were black, but so were a lot of the kids in my school and neighborhood.
That was the only time anyone attempted to touch/braid my hair outside of a sleepover when we all did each other's hair (with prior consent of course).
Key things:
1) We were all kids, and kids sometimes do rude things without meaning them that way. I've heard of grown white women reaching out to touch people's hair, though -- they should know better!
2) If they'd grown up with people asking to touch their hair and/or doing so without permission, they might not have realized that it was a boundary that was acceptable to have. Given how much it weirded me out to have it happen once, I don't doubt those who have written articles about the damage it does to young girls to be expected to submit to such requests routinely -- it's early education that they are not allowed to control what happens to their body.
3) It probably would not have happened even once if I had hair that was a normal color. By me stating my personal experience, I'm not attempting to suggest this is something white women experience often, or excuse the level of rudeness it shows.