You would be far more surprised if Google took away your username than if Twitter did. That's the point. Twitter should be held to the same standard that we hold Google to on this matter.
Why should they be? Let's say you have a GMail address, and you use it as the verification email for your bank account. If Google takes your GMail address and arbitrarily assigns it to someone else, that creates a big security issue: now this other person can (for example) get a password reset link sent for your bank account to their new email address.
If Twitter reassigns your username to someone else, it sucks for all the reasons described in the blog post, but I find it hard to see potential security issues or the potential for identity theft. Confusion, yes, but hardly anything more serious.
So yes, I would hold Google to a higher standard than I would Twitter.
I suppose also an examination of their respective Terms of Use documents might shed some light on what you can expect and the standards you should hold, as well.
