I understand what you're saying, but once you throw the word "legal" or "illegal" in there, you're playing in the complicated world of copyright law, and there's not enough case law on this issue to say whether upgrading via an .ipsw file in iTunes is illegal or not.
It's most definitely not a simple matter of "you aren't abiding by the Apple user agreement and thus it's piracy". I don't think it's wise to assume the parameters of legality are limited to those two instances.
If anything, copyright law jurisprudence has moved away from being a tool by which companies can enforce restrictive business models. The registered developer pay wall may be irrelevant to the analysis because ostensibly the fee is for a myriad of developer tools unrelated to the iOS beta. Things might be different if there were a NDA, or if the final iOS 7 release was a product that Apple charged for, but neither of these is the case.
In any event, it's enough of an unsettled area of law that you can't tell people they're doing something illegal by upgrading to the beta and do so with a straight face. And beyond that, this type of case will probably never reach a court because any case on point would involve a company coming down against a user of one of its products in a manner that would generate nothing but negative press.
