I thought dragon riders couldn’t be…

SPOILERS FOR ONYX STORM

I thought that the whole concept of venin was that they couldn’t channel from dragons or channel from griffins, so they got jealous and channeled from the earth. So how is it possible that dragon riders can become venin?

I’m getting more and more frustrated that this series poorly explains its magic system. It’s also annoying because violet KNOWS how the magic works because she’s supposed to be some super smart scribe so she could totally explain it.

Did i just miss an explanation somewhere? Or is the author just pulling straws in order to try to make something interesting.