If it's "Open" beta why do you have to sign up anyways?
Because the sign up is OPEN. Tomorrow it will be changed to CLOSED. Also, if this were a Closed Beta test (in the terms you are thinking of) there wouldn't be an announcement for it and I would just be secretly inviting people. Not unlike the last beta test where a select few were invited.
So, do you want in on this or not? Seems to me with all the time you have to nitpick the way I chose to do an announcement you could easily help us with testing.