The Imperial Liberation CB should be restricted for exactly that reason; balance. It's an incredibly OP cb. The emperor gets practically free wars on everyone in the HRE (and there's nobody there to stop him). It should also be noted that the HRE minors would likely be pissed if the emperor went around annexing people excusing it with "liberation".Most CBs allow you to do many more things than implied – the CB is just an excuse. The coalition CB is really the only restricted one, but that's due to balance concerns (a lot of allies). I don't see a similar reason for restricting imperial liberation.