You said this in your OP:

I'm not superbly protective of Sonic as I know he can defend himself, but it's like when someone messes with a bear's cub

Maybe next time, put yourself in that guy's position. Maybe his dog isn't the type to defend himself. Maybe he didn't want to risk the chance that his dog would get pissed-off enough that he'd bite your dog. Kind of strange that you'd be so protective of your dog, but not understand someone being protective of their own.

I don't agree with him raising a hand to your dog, or using any object to try and discipline him or any other dog - but at the same time, you waited until after your dog started to show fear of that guy before you even stepped in or spoke to him. By that point, from the other owner's perspective, he'd probably tried everything he could to get your guy to back off, and all he saw was you allowing it to happen.

My dog is very passive around other dogs. He just wants to bounce and play and sniff butts. If a dog humps him, he's probably going to tuck his tail and run to find me. If I saw a dog owner being negligent & allowing their dog to continue to hump mine - even if mine isn't showing any signs of being unhappy - I'd be the one confronting the other dog owner. That sort of behaviour puts everyone at risk for a dog bite.

You're lucky that you're not the one who got yelled at, imho. You put your dog and the other dog in a risky position - you say you know "which dogs do & don't play well" with Sonic, but you do not know the other dog, and you do not know which one will decide they've had enough of the humping and will snap at your dog. If Sonic had been bit, it would have been entirely your fault, but the other owner would likely have been viewed as being at fault. You should have kept Sonic away from the park if he wouldn't stop humping.