To answer 1, when V4 was queen and the same file formats were shared between the two software, Rendo posted more Poser merchandise than it currently does now. As a matter of fact, the content was equally compatible between both software so it didn't matter where you bought content (Renderosity, DAZ 3D, RuntimeDNA, Renderotica, etc.), it all worked in Poser and DAZ 3D.

To answer 2, everyone wants a piece of the pie, and ATM, the Genesis series of models is very popular. It makes sense to offer diverse content at both places. Some users don't care about the DAZ's download policies and don't want to deal with the website, in general. Some vendors don't care to deal with DAZ either for whatever personal reason, so they sell their content at other outlets.

Trying to make Poser compatible with DAZ Content is a tall order. You are talking about a major change in the application that had compatibility before. It was already tried with DSON when Genesis was first introduced, and now it is barely supported. To be honest, implementing those type of changes could make Poser unaffortable in the long run, pricing itself out of the market. And ATM, DAZ does not inform anyone when they make changes to their software and formats.