I agree that forcing them to pick a free culture license is the wrong approach, but forcing them to in some way acknowledge that their choice is no licence rather than merely leaving one out by inaction (and providing some guidance on what the choices mean) would help a lot. That said, my original suggestion was for a warning for just such reasons.
BTW, it's especially bad for for-sale assets. You really have no idea what you're actually buying. That opaque zip file could theoretically include some non-commercial license making your purchase worthless.