For the record, Blaze did finally get formal permission from Sik (he doesn't post on Sonic Retro anymore but he's on twitter) to post the No Flickes and Super Sonic in Sonic 3D Blast mods, and he's updated the descriptions to reflect that. Sik doesn't use steam and only uses Linux so there's almost no way he'd upload the mods himself anyway.
I will say in my personal opinion that if someone's just straight up gone off the grid and there's no way to ask permission, then submitting it to the workshop is okay. But that's a rare exception, and most of the time these creators are still active online somewhere.
At the very least, all of the top mods seem to be uploaded by their actual creators. Some guy uploaded Sonic Classic Heroes as soon as the workshop went up, but Flamewing told that guy to take it down and now Flamewing himself uploaded it. I'm willing to bet we'll see more instances of that over the next week or so.
Er, I'd say maybe don't publicize this sort of stuff. It is definitely illegal and violates both GAF's and the Steam Workshop's TOS.