If you're a sponsor then I don't think there should be an issue with them doing group purchases or discounts, or promoting their products anywhere. These sponsors have already paid for the ability to advertise their products. If I paid money to sell my products on this website and then I can't run promotions or whatever, I'd be like "******?"
As much as I may disagree with not letting members ban together for discounts on products from nonsponsors, I can see why the site owners may choose to run their business the way they do. Their site, their rules and all. I don't hafta like it, but I can see where they are coming from. I do think that allowing members with no business interest to share deals or "finds" should be allowed though. That's one of the great values a collaborative forum can provide.