Actually, it was a major change. As you said, locking maps in WC3 required the use of third party applications and as such it wasn't a very common practice, especially in the beginning. In SC2 this was present from day one, and as such many if not most developers opt to lock their maps. On top of that, the new Battle.net platform makes it far less common for players to find and share the map files themselves, since they are readily available in the client and do not require you to get a map from another player.
I have nothing against people who want to keep their maps locked. All of mine have been like that since the beginning too. This is a completely voluntary effort.