The thing about it is is that they can change the law if they want. You have to remember that they were on their own in the galaxy until quite recently. They have been dealing with rebels since well before first contact aswell. The thing that keeps the general populace in line is the culture the captiluim has shaped over thousands of years. It also helps when you don't feel anything for races that arnt your own. Another thing to keep in mind is yes it is influenced just it is influenced my by the cult of personality surrounding the sovereignty's exsistanse. And yes rebels have always been a problem which they consider worth the price.