I also get right to rule when under merc contract. So far I've only ever been under merc contract to the Rhodoks.
You see, the Rhodoks are so badass and so very much elite, even their most lowly soldier can be considered fit to gain credit to rule over the filthy masses which make up the pethetic upstart rebels of Calradia. You see, for the entire time I have been contracted to my one and only King Graveth, many rebels have risen against my justful authority, of which they were soon to take wisdom in the real position of their worthless lives and would make peace with my almighty King.
Should you gain right to rule under any faction other than the Rhodoks, then you my friend, are a blaspheme and should be put to work in my glorious Rhodok-ian taverns as a... Uhhm... Table Dancer!