Shoutbox

This is a limitation, sorry.

These modifications to add buttons are made to Messenger when Messenger loads its resources, windows etc (thats also where it applies skins). All necessary resources are loaded when Messenger starts, before you have signed in. Since you aren't signed in yet, it isn't account-specific.

Edit: The same thing is with polygamy, because when Messenger starts twice, MsgPlus! has a choice whether to let it run or close it. MsgPlus! has no idea what account you are going to sign in with. That's why it's a global option.

Edited: the post is true but there's no relation with advanced technology here, just choice of design