I like the idea of "going green" but some people like having a physical copy to refer to. Which is why I bought the German Bond museum catalog. Many of the images in it are, to my knowledge, just not available on the net. I like having them all together, in one place, to refer to from time to time without turning on the computer and burning up electricity.
Just about all the auction houses watermark their images in some way, which I find distracting. And even if you downloaded them as a pdf, you could still print them out, which dashes the "go green" concept. You can't hang a pdf on your wall. I know it's not the original poster but if you can't afford the original, a well printed repro will do just as well. And yes, I know many collectors who find that highly distasteful and disapprove of it strongly.
I have no idea what the printing, binding, and shipping costs are for catalogs but they may be a loss leader. Heritage has always charged $50 for their catalogs so the cost for them to produce them is maybe a couple of dollars a piece. What happens to the extra copies after the auction is over I don't know. They may be sold to someone who then sells them for much less on the open market. They may also be disposed of by shredding them, thus returning them to the mill to be ground up and made into new paper. They may be put into storage and kept for a while in case someone wanted to purchase a copy of the catalog at a later date after the auction was over.