Why is Mordekaiser listed as Thresh's ally but not reciprocally?

Mordekaiser doesn't have allies, he has subjects.

It's likely because Morde holds some type of control over Thresh via the Black Mist that Thresh is listed as his ally.

hecarim and karthus are his allies. I can see why karthus would be an exception since he embrassed death but i don't see the difference between Hecarim and Thresh