I would expect that the "totally wipe my phone" process would include an API call to Apple to disable iMessage for that phone so people don't have to make special calls.
Sounds like that doesn't happen.
Of course if you just throw the phone away without ever doing something to indicate it's no longer in use (to Apple, not your cell carrier) then the results are more sane. That can't happen to every one of these people though.
If my phone is stolen, so I wipe my phone via MDM. I still need to communicate, so I can use my Mac or iPad to do so transparently to the other parties.
This is a common issue whenever you do caching. Outlook fubars contacts whenever you migrate Exchange users to or from a system.
Sounds like that doesn't happen.
Of course if you just throw the phone away without ever doing something to indicate it's no longer in use (to Apple, not your cell carrier) then the results are more sane. That can't happen to every one of these people though.