Because it sells, and everything that sells on this site supports Poser whether it's meant to be used in that program or not. The vendors here aren't freeloaders, a hefty percentage of what you pay for each product goes to Renderosity. They feature what brings in the money. The vast majority of vendors include enough information to discern which program it's for in the product description, and some even include it in the promo thumbnail, so finding out it's for Daz *after* you click on it is easy to avoid. But like Rhia said, it's fairly easy to filter out, in multiple ways. I certainly don't think that stuff for Poser is a dead art, because a bit more than half of my "the artist you're following released a new product" emails are for Poser.