I've heard it explained that "hey" used to be more of an urgent way to get someone's attention, rather than a casual "hello" like it is now, so it sounded rude to some older folks.
In the '90s I had a snap back that I decorated with stick on jewels (beadazzled) and on the back I wrote "YO!" in sparkly silver puffy paint. I thought I was so cool in that hat.