User blog comment:DaNASCAT/Technical Update: February 16, 2012/@comment-3279487-20120218134221/@comment-13301-20120219112808

And view source code is an option that needs to be marked in Special:Preferences, otherwise it's not available.

I've never unmarked it, that option was unmarked by default, so I guess the majority of users are unable to see the source code of a wall message.