I thinks that's easy to answer... why sould the SD suppot, support a beta.
It doesn't have to be a problem of the game... There are still bigger problems than looking into problems with a non supportet OS that isn't even retail.
If you mean the netcode, those are two very different teams handling it.
I'm not saying they should support a beta OS, Windows 7 is not beta anymore, it's RC, but I won't go into semantics. No company will support beta software officially, because in certain countries that gives the customers rights. However, that does not mean they shouldn't be looking into it yet. It's what TechNet was invented for, so devvers can learn and test their code on beta OSs so that when Windows does go retail they have a fix for the problem already. Every devver knows this, and so does Stardock. The fact that Windows 7 is not retail yet does not mean they shouldn't be looking into the problem.
I still agree that the TS should try it on another OS first. But simply saying "we won't look into it because W7 is RC" does not make sense.