If you send it in a message to everyone, then everyone knows about it. That's the ground rule. And it's the only ground rule that can possibly work in an MMORPG like we have. Maybe you could make it work otherwise in a smaller group of friends or private gathering, but not in the open environment we have in BattleMaster.
Now, how a person knows something, and what they do with it is an entirely different issue. But if you RP that your character is thinking rebellious thoughts, then I wouldn't expect someone to reply to that as if it had been an open public conversation. But I would think it perfectly reasonable that others could observe that your character is unhappy with the government, increasingly discontent with the ruler's actions, etc. Because if you didn't want me to know, then why would you be directly telling everyone in the realm that this is what your character is thinking?
The alternative is for people to continually be rubbing your face in things that you have to remember that your character doesn't know, but you as a player know. Like, for example, assume that I had an infiltrator character that wanted to assassinate a duke of the realm. If I sent several RPs about my character running over plans in his head about how he was going to kill the duke, would you expect the entire realm to just ignore it? If I went to try the hit and saw that for the next several days there were people out patrolling the streets to make it harder, should I start screaming "Powergamer!" because there are police patrols around when there really isn't any valid reason? (Or at lest one that I know of.)
Or how about if I RPd my character thinking about the coming rebellion, and listing the names of my conspirators, and how we're going to get the army sent away so the plan works easier. Is the entire realm supposed to just go along with the orders, knowing full well that as soon as they get four regions away for the capitol, that the entire home guard army is going to rebel?
Can you see how this kind of thing is entirely unfair to the rest of the players of the game, making them try to remember to separate all this information that the player knows but not the character? You're forcing everyone in the realm to just have their character blindly go along with this, knowing that their characters are walking into a trap.
And then let's not forget that you /always/ have the opportunity that someone just innocently slips and uses the info IC because they forgot that the information was sent as private thoughts. Then how do you resolve those kinds of things? Especially if the slip happens when the "thinker" character doesn't see that the information is being passed. Like it gets sent to a foreign ruler. And then that foreign ruler thinks that the info is perfectly valid IC info and spreads it around.
And lest you think I'm just making this stuff up, I've personally seen some of this stuff happen, and heard of more.
