Difference is they are in no position to rebel and besides would the horde care if she did go full on lich king? I mean didnt Vol'jin once threaten a ally player that he would feed his body to slyvanas? Clearly the horde knows what she does and doesnt give a fuck.
As long as she doesnt affect the horde way of life which is just to survive they aint gonna go after her just because she can rez and control the undead which she has been doing since garrys time!
Had the Lich King not attack orgrimmar and had sylvanas not had a beef with arthas iam sure the Horde wouldve stayed home during the whole northrend thing!