There's nothing illegal about contributors benefiting from their donations to non-profits. Pharmaceutical companies regularly donating to various illness based charities expecting it will increase the sale of their products is just one example. The networks are going to have a hard time proving that Locast exists to benefit the sat and cable providers in negotiating retrans agreements unless they can come up with a paper trail to back that up. That Locast happens to benefit the sat and cable providers along with ordinary viewers is not going to cut it. They'll also have a tough time proving any other benefit to the sat and cable providers, especially in Dish's case where the company takes a subscription price hit when the sat locals are dropped in favor of using Locast. It wouldn't surprise me at all to see the sat and cable companies either directly or indirectly contribute heavily to a Locast defense fund.
Ok, I did say I don't see where that is prohibited, and I still say who knows what a Judge would say. In any case this is small to the rest of my post of why I think the networks have a chance to win in court.