Why do think there was such a serious punishment for mutiny?

Well, sweetheart, mutiny is basically a fancy term for a bunch of sailors deciding they're over their captain's crap and trying to take over the ship. Can't have a bunch of salty sea dogs running amok, now can we? So, to keep order and prevent chaos on the high seas, they had to come down hard on those rebellious rascals with some serious punishment. It's all about maintaining that delicate balance of power and discipline onboard, darling.