Any map, even protected maps, can be found in the Battle.net cache folder (C:\ProgramData\Blizzard Entertainment\Battle.net\Cache on Windows 7/Vista). If you open the map in the editor, you have access to everything in the map, except you cannot see anything in the trigger editor. But this does not really matter because the MapScript.galaxy file can simply be extracted from the map. You don't even need to extract it with an MPQ editor either, since you can just extract it with the editor's Importer (you have to check Reserved to see it on the list). I believe you even get to preview it in there too.
So if you decide to open somebody's published and protected map, searching through your cache folder for it would probably be a lengthy process. To make it easier, delete or backup your Cache folder, start up StarCraft II and download only the map that you want to open up. Search your new Cache folder for .s2ma, and search through a much smaller number of maps. You can also see the file size when downloading the map, so that also narrows down which ones to open.
Regardless, it is very easy to open and edit maps that people published and protected. No harder than WarCraft III, just a little more time consuming (also, no one has made any kind of map protection for SC2 besides Blizzard's, which obviously doesn't work well).