it's not the same. SMS is something that after it is sent is no more related to any user but here in a forum where everything is stored in a DB why not allow editing a PM?
A PM, as well as an SMS, is a direct message between 2 (or more) identifiable persons (or terminals). This opposes to a forum as the messages posted have no direct targeted destination. Any one on the forum can answer.
Let's exaggerate a bit...
If I post a thread saying something like "you son of a xxxx I am going to kxxx you", this cannot be used against me because I targeted no-one;
But, if I PM you the exact same phrase, I am targeting you and it can be used legally against me.
btw whatsapp allows deleting msg after they were sent it would also be cool if they allow to edit them.
Even SMS carriers allow you to delete sent messages... on Your device ONLY.
The terminal to wherever the message was sent still has it!
Same goes to whatsapp.