Using everything I've laid out someone 'stealing' a map and changing it for whatever reason, it won't matter at all. Nobody will play it.
The person who "stole" the map will play it, as will those who join his game (either naively or inattentively) and automatically download the modified map. Also people who don't bother to check if the version they're playing is made by the "real" author.
I'm not arguing against you here. I totally agree a community-driven process is the only real solution. However, I still think there are three good arguments why a read-only flag complements it well:
1) As mentioned, it gets rid of little kids with little sophistication who may flood both the database with bootleg versions of the map, a convenience for everyone involved.
2) Regardless of what community-based solutions are approached, some people will want map "protection". If Reverie doesn't offer it in their map editor, they will find a way to achieve this using third party programs.
3) It makes it explicit whether a player approves or disapproves others modifying his map.