Rob, the whole thing is really a grey area, and some would accuse me of stifling the free-flowing exchange of ideas because of our policy. I disagree.
What you're bringing up about nurseries that don't sell orchids can similarly be applied to orchid vendors who don't sell the same genera of orchids we do, can then be further refined into orchid vendors who sell the same genera but not the same clones, can be honed down to maybe the same clones but not the same sizes, etc.. So, rather than getting into all of that, the policy remains that no advertising for any commercial vendors of anything--can openers, awnings, pumpkins, anything--is allowed.
Several members who have been here since very early on (uncasteeb, for instance) have gotten along just fine and contributed immensely without making the first mention of any vendor whatsoever, so doing that is completely possible without sacrificing anything in terms of valuable content. Instead of saying "Got this today from VendorX!" you just have to say, "Got this today! Isn't it fantastic?" If someone wants to know from where, he or she can PM.
Finally, context will determine what constitutes education and what constitutes advertising, and Bruce and I have the final say. Some people, thinking to be very clever, have tried to disguise one for the other, and that won't fly either. If questions about cultural recommendations come up whose answers can only be found on websites for commercial sources, those answers are easy enough to paraphrase. If someone wants page and reference, use PM.
There is really no difficulty here that can't easily be worked around.