Many people have brought this issue up, myself included. However, I do believe the relationship penalty is working as intended, as you are breaking your mercenary contract by quitting early, and this leads us to the real problem... The mercenary contract goes on indefinitely without ever ending. I believe you are suppose to let the contract run its course, (given the contract term is coded properly and working, which at the moment it doesn't seem to be) and decide if you want to renewal your mercenary contract or not, with that particular faction. If you don't renewal, then it make sense for you to quit and not have any relationship penalty, but if you quit while still under contract, I do think you should be hit with a penalty, though maybe not as severe as breaking your vassal oath, something like a -5 with all lords at the most, since to some people's points, mercenary shouldn't be regarded as that loyal, outside the confinement of the contract anyway.