It would be considered too easy but they should have gotten around it other ways (like city unrest, for example). As far as I know the couthouse is the only normal building you can't purchase, which makes it stick out like a sore thumb.
"I'm designing a game where you can buy almost any building in the world instantly! Libraries, Universities, factories you name it!"
"Courthouses?"
"Oh...uh...no. Sorry."
"Why not?"
"Well, you see....Courthouses require special bricks of law-giving and order that can't be bought or....something...."