It qualifies as a negative review because it's almost always accompanied with 1 star. 1 star reviews matter in the AppStore.
I'm sure there are bugs in the beta or bugs in your code that really need the new SDK to fix, but I suspect that the vast majority could be fixed or worked around without too much trouble. Maybe your grand iOS 7 redesign is going to be the greatest thing since sliced bread, but you can likely bodge a fix together in the meantime.
A large part of the point of Apple releasing betas early to developers is to give them a buffer time to get their apps ready. It's unreasonable to demand developers "bodge a fix together" for immediate use as soon as a beta lands, it defeats a large part of the purpose of having these betas.
I can think of a few niche reasons why you would wait until iOS 7 is released before you release a fixed version of your app, but for the vast majority of people, I think they ought to be working on getting a version that runs within a few weeks of seeing a beta OS version that crashes their app.
Also, while adding a catchall preventing beta OSes from reviewing is good, I have seen other posts advocating mentioning the iOS 7 is no compatible in the description of the app. This is worst advice ever. Those who are not developers who are using the beta cannot be reasoned with and it is their own fault for having an unstable OS. Mentioning iOS 7 will further annoy them [that the app doesn't work on iOS 7] and confuse legitimate users about the availability of iOS 7.
I still think that the bigger issue is this: if so many of your users are doing this that it's a problem, then fix the problem. If you want to complain about "lusers" in the bar later in the evening, you have my sympathy. But if this is a real problem for you, then it's an even bigger problem for your users, and just calling them unreasonable doesn't help either of you. It might be their fault, but they're still your customers and they still can't run your app. The 1-star reviews are not your biggest problem at that point.
If 50% of my users get a crash at start-up, I'm going to try to make that go away. If no one is really having the problem, then it's silly to ask for Apple to block potentially legitimate feedback. If loads of people are having the problem, then the cost-benefit analysis ought to look very different.
Beta software is exactly that -- beta. Users should expect failures and problems, so giving any sort of review based on iOS7 beta software is absolutely ridiculous. The funny part is that the only people who should have the beta, should know better. But obviously they don't. I wish there was a way to lock down access to the beta program to legitimate developers, but that's impossible because it wouldn't necessarily be a fair system. Although it could be possible to restrict beta access to developers who actually have a released app in the store. New devs could still use the existing SDK and tools and submit, but they wouldn't necessarily have access to the betas. I don't know what problem that would solve, however. It doesn't seem to be a significant issue, except in app store reviews, so the simple solution is to restrict reviews to non-beta people and instead of reviews, iOS7 users can submit private bug reports. This way devs are aware of problems they might not know about but it doesn't give the negative publicity that affects their sales. Several one-star reviews can kill a newer app before it even gets off the ground.
One potentially cool alternative is to allow devs to release iOS7 beta versions that are only accessibly to iOS7 beta users through the app store. However just as promo-code apps can't be reviewed, the same logic should apply to iOS 7 beta users.