Yupp, although it's possible to deprotect maps, as far as i know you're mostly allowed to add stuff, but not all triggers, etc. are viewable/visible? So you could editing maps, but not steal/copy triggers from it? (And those few who are able to deprotect them correctly with all triggers, etc. are not many)
And as most said, this might be the best option for protecting:
http://www.wc3c.net/showthread.php?t=79326
As minimum you should check the "editor only parts" to be removed, with this the map will give erros, when somebody tries to open it. Beside of that it will create a "new" version of your map, means you can keep the old/original version that is unprotected.
If you keep the .j file (or whatever it was, that will be created, too) I believe you also can deprotect the protected map. But I'm not sure about that (you could erase this file, if you have your unprotected map, I guess it stores the removed editor parts)