I think many people here are looking at this the wrong way.  This isn't a matter of art vs. non-art, it's a matter of craftsmanship. Arranging pre-made figures/props/textures in an aesthetically pleasing manner certainly requires skill, but it requires more skill to do all that plus make all those figures/props/textures in the first place.

In a production environment, the end result is all that matters.  Get the job done on time and make the client happy so that you can get paid.  If pre-made content will help you to accomplish that (and the license agreement for said content allows it) then use it.  However, the websites that show the worst anti-Poser snobbery are meant to be showcases for those who create their own content (even if the site doesn't say it right up front).  It’s not just the finished image that's important, it's the work that went into making it.

IMHO, painting vs. photography is an inaccurate analogy, because they are vastly different mediums.  Painting is the art of inclusion, while photography is the art of exclusion (I can’t remember who said that).  Perhaps a better analogy is fine scale modeling or model railroading.  If you bring a model built from a kit to an exhibition or competition for scratchbuilt models, you'll get flamed.  It's the same with pre-made vs. self-made content at those websites.

That being said, I do think those websites should find a more polite way to tell you that you’re not welcome.