"I think a lot of these artists, especially the new ones, just want to use your art (from their mesh) as part of their portfolio."

Could be something like that. I know if I saw something of mine used in something very public I wouldn't hesitate to point out if I made a texture, etc. Nothing wrong with that.
That said, such a restriction isn't possible in a product in the marketplace because it contradicts the marketplace license. It would be better as a request and is entirely reasonable as such. But not as a restriction.

...... Kendra