As far as I can tell, all of the heal spells generate the same amount of hate per HP point healed.
So: Regen doesn't create any more hate per hitpoint compared to throwing a Cure. It's just that if it's on the tank and he pulls a group of mobs he's usually only hit one of the of them. If the regen tick happens before he's managed to land any damage on the secondary mobs that healing will put you onto the top of the hate table and they come running at the healer.
If you timed a Cure to happen at that precise moment the same thing would happen...except it'd be worse, because Cure would "heal" more HP so you'd end up with more enmity than a regen tick would cause.