Whoever made that video was talking nonsense when they said the relationship is "dangerous". Stingrays are pretty intelligent critters, and since that one clearly isn't afraid, it's not going to sting them unless somebody is an idiot and jumps on top of it or something.
My guess is they fed it a few times (possibly by accident) and it learned to come for food, then it decided that it likes the petting. I wonder where this is?