Facebook, Twitter, Skype, Gmail, etc. whether they charge users a fee or not, all present themselves as basic infrastructure services, with an implied expectation that if you register a name with them, that's as good as having a phone number, and it will not be taken away from you unless the service is discontinued.
Legally speaking, Twitter is probably okay here, but if Facebook or Google did the same thing, I would expect they could be legally liable. Google's TOS for example states:
>We take appropriate security measures to protect against unauthorized access to or unauthorized alteration, disclosure or destruction of data. These include internal reviews of our data collection, storage and processing practices and security measures, including appropriate encryption and physical security measures to guard against unauthorized access to systems where we store personal data.
It's reasonable to assume that people are using Gmail as a verification email for other services, so they could actually be breaching their own TOS by giving someone's username to someone else, since when you've used a Gmail account for even three months, it becomes personal information that should be protected.
Ethically speaking, all services should be doing due diligence when reassigning usernames. This isn't IRC, all of these services are marketing themselves as identity gateways, and reassigning identities without cause borders on outright fraud. In this case of course, they're actually acting as they normally do, trying to make sure that online identities match legal identities, so it's more understandable, and probably protects them from allegations that they're facilitating fraud. But your authoritarian system is ethically dubious, and furthermore unlikely to hold up in a court of law.