actually, they don't - not at first. You can get to the account and write a tweet or two or many many for as long as you are logged into that session. it's when you go back that they expect that you've clicked on the confirmation link they sent. And you can't ever get back to that account otherwise because, while you started the process and maybe even tweeted, the second login requires the validation email link to truly complete the registration.
The reason they do this is because they want to suck into the max number of users. The important part is to get you into your account prior to confirmation so that you can start networking with your friends and start following people. Once you get that ego boo, and once they've collected the important info about networking (which the reason why these sites want you to be there in the first place: to find out who's like you, who you hang with or want to hang with (follow) etc.), they figure you're more hooked, and committed than if you had to wait for the confirmation link.
Happen to be rebuilding a shopping cart right now, and it's precisely the route we're taking. We "hooking" them first, then worrying about the details later. it's a new approach to the user experience. Except, of course, since we know better, we're going to use ajax to at least validate some aspects of the email address off the bat (e.g., top level domain, syntax, etc) and require duplicate email address entries to capture mistypings first before they get through. In this case, the dumb asses at Twitter don't even validate the TLD. you can "register' with the email address userfoo at gmail.orn and they don't even catch the error with a TLD check. I tried it and it validates, which it shouldn't. They don't ask you to retype your email address as a validator either. Dumb.
Lack of experience is the cause of that glaring error. And the separation of software dev from network ops. The network ops folks are cursing the bounceback email, no doubt. But since the software devs never learn of their mistake, they never learn to be better devs in outfits like twitter that probably grew too fast for their own 'good'.
-- http://cleandraws.com Wear Clean Draws ('coz there's 5 million ways to kill a CEO)