Because it's entirely logical for me to honestly say that I want tokinvade every country on the face of the earth and go off on a maniacal rant about it in front of everybody and be completely serious about the entire thing.
I'd rather not have to hold my tongue because it could have in-game consequences. That's just me.
That, and...
OOC comments in a game thread must be clearly marked as OOC.
Since there's no way to prove that something said in the chat is in-character or out-of-character because the chat
is not an in-character chat most of the time, there is no ethical way to use screencaps from the chat as proof of intention in the game, which is why most GMs don't allow screencaps in thread.
The only way screencaps of chat can actually be posted that doesn't skirt the rules is by posting OOC tags on them, but if other players take diplomatic actions based on OOC comments in the chat, then any player doing so can and should be considered metagaming.
And really, even then, screencaps wouldn't be allowed under the following rule.
OOC posts in a game thread should avoid mentioning other players by name except in a positive light.
Since we've established that IOT Chat is, by nature, OOC, and that screencaps are, by nature, OOC, and that OOC posts in a thread have to avoid mentioning players by name except in a positive light, then we can reach the conclusion that chat screencapping and posting in thread is scummy, meta, and probably illegal within the confines of the IOT Constitution unless the specific IOT has a specific chat dedicated to IC diplomacy.