@JWChibi: Considering he's THE Emperor, if he want her in prison he can easily do it without needing a legal threat.
He's just doing it to make her more likely to accept the fiancee offer, since that's their real goal.
Similarly, if he want to make her his fiancee, he can totally force her to be one, but given what they want (a 'bait') he want her to be willing to do this.