The "Page information" page for any message in the MediaWiki: namespace says that the message can be edited and moved by all users. For example, "MediaWiki:Wikimedia-copyrightwarning" exists, so http://en.wikipedia.org/w/index.php?title=MediaWiki:Wikimedia-copyrightwarning&action=info says "Allow all users" for both rights. For a message that doesn't exist, such as MediaWiki:Is awesome, the info page says that all users may create the page.
Example from Wikidata:
For example, at https://www.wikidata.org/w/index.php?title=Property:P7006&action=info (a semi-protected property), the page information displays:
This should clearly indicate that the page is semi-protected by $wgNamespaceProtection (as all properties on Wikidata are now semi-protected, see T254280). Apparently this is a generic issue with $wgNamespaceProtection, not something that is specific to Wikidata.