As someone who mains a tank, I get equal commendations on dps and healer classes as well.
Yes, I will admit that if it's one of those static nobody says anything, in-and-out kinda parties, I usually see myself gaining those 'defaulted' commendations.
However, a {Hello!} can go a long way. As long as I don't royally mess up a mechanic somewhere, I tend to garner commendations regardless of my class.

However, in regards to the idea that tanks and healers get them more often, I don't think that's such a bad thing, nor something that needs to be fixed. You see, we lack many tanks and healers in the queues more often than not. We're not often short of dps classes (though I've been there!). Just being thankful that the tank exists in order to allow the roulette to happen is often enough.