The reason stated is that they do not perform with V4L juice (and they advised V4L not to use V4L juice).
It's like Altria turning down empty cigarettes because Marlboro tobacco won't work in them. That's certainly a valid reason for Altria to not want the empty cigarettes. They also indicate to get ONLY Dekang or their only other approved juice Ecopure. And they make 3 kinds of Dekang, the PEG is stated to produce more vapor than PG.
What?! you can't be serious?