Game access levels
I would say that the difference is that every GM in a game is as responsible for the game as the GM-owner is, so if they screw something up, they're all equally responsible. There is no 'well, the co-GM did it' argument when it comes to the rules. A GM is a GM is a GM. (Historically, if one GM of a game broke a rule, for instance, all GMs of the game were punished the same...this may no longer be the case, but it certainly used to be.)
GMs are responsible for policing their own game/players, and if their players have the ability to essentially write posts using One-Off names and the GM then can't tell which one of their players authored a problematic post, they don't know who to come down on/remove so that they and their entire game as a whole doesn't get modded.
Since GMs are responsible for all content within their games, they need to be able to tell who wrote that content. (Presumably, if you have a co-GM, it's someone you trust explicitly in a way you probably don't your players, and if they wrote one-off posts, you should know it - if just because it wasn't YOU.)
Which comes back around to somehow tagging One-Off posts with the actual author so that GMs could tell who wrote what, and if you don't mind knowing who the actual author of a post is, there have been several suggestions above that would allow players to post "NPC posts" using their actual characters. Equally, there have been several suggestions that would allow your players to write "NPC posts" using a character that other players wouldn't know was theirs by giving them a secondary (or even tertiary) character to control for that purpose, while still allowing the GM to be 100% aware of who authored what.