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.
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.
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.