You know, I was thinking about asking this question myself. My HM's tail literally drags him down. It's so big, he can't even begin to open it when he flares. He's not an OHM, he's more like a freakin' FULL MOON. I actually would be thrilled if he would bite some off but he never has.
My thought is this: can it really hurt that much for a human to trim the fins? I mean, it can't hurt any more than the betta chewing them off himself, right? As long as the water is kept clean after the fincut, there should be no chance of infection, just the same as if the betta had bitten them off himself.
I think if you want to go around trimming fins for cosmetic reasons, to make a "designer" betta, then you're pushing some ethical limits. But if you are trimming the fins to help the betta, then I dunno, it doesn't seem so bad.