Apple releases iOS beta versions in stages, usually about every two weeks through the summer until the new iPhone is announced in autumn. This year there have been six different betas, with maybe one more to come before the Gold Master that is the final version before the new iPhone is released. These betas come with expiration dates, and if you are still running an old beta (like versions 1, 2, 3 or 4 in this scenario) when Apple lets it expire, then the device will stop working.
Apparently, many of the people that have downloaded the iOS 7 beta (developers and non-developers alike) did not follow along with Apple’s over-the-air updates of new beta versions and subsequently got locked out of their phones. With so many non-developers using iOS 7 beta, this caused quite a ruckus.
While letting old beta versions expire is standard operating procedure, many users reported that the Apple activation server was down as well meaning that people could not get back into their phones without completely resetting it and downloading the latest version of iOS 7 (beta 6) from the developer portal. While it is unlikely that Apple was going after non-developers using iOS 7 beta, letting their phones brick was perhaps an intended consequence.