Axe, sweetie, you just about had me go into a fit! LOL!!
I have a DA account in which I supply renders as stock for photomanipulations....
Several months ago, I stopped because this very issue came up in the Daz Forums. I watched the topic carefully, even asked if what I was doing was wrong & if it violated Daz' EULA. SKrondis himself said he doesn't have any issues with "tubes" (if I could hunt down that long, long thread again....).
The issue has more to do with rendering say textures on a plane or square where someone could extract them from a 2D image and the Daz Environments. Straight renders of the Cyclorama & other environments are a strict no-no, even if you throw in a couple of plants. The environments have to be filled in such a way that someone can't get the the actual Cyclo, MilEnvironment, & Playsets.
I have started doing my "3D" stock again BTW and I am careful about how & what I render. I don't promote my DA stock to 3D sites as they are for 2D manipulations or web design.
If .pngs or "tubes" are not allowed then there are a slew of DA stockers & tube providers on the net that are in a heap of trouble, including myself.