The whole point of muting someone is to remove them from your Hubski experience. If I've muted someone, I'm not interested in conflict resolution, I'm interested in not having to deal with that person. By allowing them to PM me, I not only have to deal with them on Hubski, but if I've got the system set to email me when I get messages, I have to deal with them during the rest of my day. It's extremely easy to contact someone who has you muted. If that person really cares, they could get a 3rd party to intercede and vouch for them. It'd probably count more anyway - If I've muted someone and they want to be unmuted, a convincing argument from someone I haven't muted is going to count for a lot more than a plea from someone I'm deliberately ignoring.
The last person I want pming me is anyone I've muted. Personally I can't imagine being bothered enough about being muted to need to come to terms with someone about it. I think if I knew I'd been muted I'd respect the persons decision enough to bother them no more.