Kickstarter backers technically haven't actually bought anything. As you say, the reward for the beta tiers is stated as "participate in the [...] beta test" and unless FD skip one of the tests or explicitly exclude people from them, they will fulfil that reward, regardless of whether certain groups are allowed to participate earlier.
That's not really the issue here. The issue is that FD weren't transparent about the process. It may be that this has only just occurred to them, but in that case I think it would have been wise to consult with the alphas in the alpha forum (or at the very least the DDF). This reaction could and should have been anticipated, and I don't think anyone in the alpha group would have felt hard done by if everyone were to gain access at the same time. In fact, that's what I as an alpha tested assumed, and I'm sure most of the rest did too. Now having said that, I can see benefits in giving access to the alpha testers first (to get the worst of the bugs identified before releasing to a wider audience) but that should have been part of the plan all along.
Once again this is borne of poor communication, and as long as FD deny that they aren't communicating well (when a lot of the evidence shows otherwise) then this sort of thread will continue to be common. It might be a case of throwing toys out of the pram or simply venting frustration, but if FD were more careful (or at least transparent) in their choices there wouldn't be anything to be upset about.