3.13 is ancient, 2 generations old now, and honestly I think it's well within the realm of acceptibility for Apple to disable new downloads. I mean, the vast majority of new apps require at least iOS4 anyway. If this needs to happen to make the appstore more efficient for everyone and enable new features, this is fine. Sure, some people will be pissed, but I think it's a very small minority of ACTIVE iPhone users who are still stuck on iOS3. Also, whats with that screenshot? I've never seen that textured look before.
I'm pretty sure the "vast majority" of apps are designed to run on all versions of iOS, unless they're graphically intensive. That way developers are sure they're offering their apps to as many users as possible.