It's not cruel in the slightest. My horse stands there asleep while I pull. Mind, we're pulling half inch or smaller sections, and it comes right out. It's when people grab giant, huge, thick sections, when the horses start getting stressed over it. I just pull the strays underneath his mane, to clean it up a bit, and make it look fluffier and more natural.
Always pull after excercise while the horse is still a little warm, and then a pores are opened. Take a half inch section of mane, tease it back with the comb so your only left holding a small amount of mane from the underneath side of the mane, wrap it around the comb and slowly put pressure on the comb until the hair slides right out. I'm not a fan of just ripping it out as fast as you can. More relaxing for the horse when you take it as slow as possible.
Again, my drama queen of a horse stands there asleep. If he was concerned about it at all, I'd know.
"You're just as sane as I am."~Luna Lovegood.