MakerBot Print is our newest print-prepration software, which supports native CAD files and STL assemblies,
allows you to interact with all your printers via the Cloud, and many other exciting new features.
Of course they don't have to give away their designs for free. But that is what Thingiverse is about, so then don't post it here. I have given away over a dozen designs here. As the pay-per-use model of 3D designs grows, imagine what Thingiverse will become as everyone tries to use it as free advertising, and you find that half the objects you find are not really complete without paying money... it should at the least be categorized as such on the site somehow. But then of course Thingiverse would want a cut of the action and why shouldn't they.
Now if Thingiverse wants to ban users who post truly free designs, and only allow commercial providers and leechers as you suggest, then then I suspect the site would die fairly quickly...
If they want to post sample parts I imagine that thet could easily host that on their site.