I seem to be getting the impression that a lot of people are instantly assuming that the Magister is in a more "politically powerful" position that the royals of true blood. Indulge me for a moment as we examine the evidence together and try and determine whether or not this is the case.
First off, I wouldn't say he's more politically "powerful" than Kings or Queens. Every government has diferent departments and a system of checks and balances, such as the united states government which is made up of three branches: Legislative, Judicial, and Executive. It's set up that way so no one branch has more power than any other, and things function as a cohesive whole.
Vampire hierarchy is slightly different, as there does appear to be an Authority (legislative) up top, but that's not to say that it's a straight ladder down all the way to Sheriffs. I'd be willing to bet King's and Queen's are on the same tier as the Magister(s), but the only reason Sophie-Anne and other monarchs might fear him is because he actually is in a position to make her life difficult via catching her in a crime, especially since his job is to correct crimes (as a member of the "judicial" branch, whilst Sophie-Anne would be "executive.")
Eric Northman's allegiance to her, especially when she had asked him to do something illegal, also supports this theory of different branches, since it has already been established that sheriffs answer to the monarchs, thus he is under Sophie-Anne. Not the Magister or the law he enforces. Since it would be treasonous to even turn on her or disobey her, this further affirms the Sophie-Anne is the equivalent of the "executive" position. As such, monarchs no doubt have some form of political power that Magisters do not, and vice versa.