Because your Sith character is smarter than you.
Not saying this to be insulting just responding with the truth, an intelligent Sith (which is what our characters are supposed to be) does not squander a valuable resource unless their hand is forced in the matter, or the resource finally gets used up. Quinn, as the story goes, has much to offer that puts him in a unique position against other officers of his level.
A treacherous swine does not equal a valuable resource no matter what his abilities. Especially in Sith world, where being able to trust someone is a luxury. In real world, any world, he would have been removed from service.
In eve online, you don't let an AWOXer who once revealed your fleets position to the enemy so they could stage an ambush back to fleet. Ever.
Im perfectly fine with there being this kind of treason among the companions, but at least they could have tried to make it so that he was forced to do it somehow, so he could have been forgiven more easily or something.