The permabans are as much for protection as punishment. It means the account won't get lost before the original owner gets the chance to appeal, and if the wrong person does try to appeal then at least we can be 100% sure they aren't the right owner and the account won't have it's ban run out and just go back to being "shared/traded" and the like.
Plus, permabans get shortened. You only see permas on shared accs when they aren't given back to the original owner. (Generally because of no appeal/claim from said owner)
The ones that get unbanned from "acc sharing" and the like have either been reduced or unbanned cause the case can range from a repeated offense to getting tricked into it.
I assume you're not saying all shared acc bans should be 2 months max, irrelevant of why the acc was shared at all? I hope not anyway, I'm not fond of letting hackers and tricksters get away with it if they just don't log in for a month or two.