Offering spammers a regulated way of reaching a mass audience also gives Twitter the ability to be much more aggressive in shutting down accounts who pose as real users.
Isn't it a trivial task to automate twitter account creation? Shutting down christinanik2883 because she didn't register as a Botified Citizen doesn't do anything about jonnyi9032 or shellystack48799823.
If you ask users "Do you want robots to be able to send you spam? Yes/No", pretty much all of them will say No.
Hence bot-owners won't be that keen on officially registering as bots.