Maybe they can't. They sure lack some core competence by not being able to block obvious spam accounts (Newly created accounts that: posts trend spam, do mass following and mass @mention, etc).
I'd love to hear someone from Twitter explain why the service seems to lack even basic automated spam detection. The only mechanism that seems to exist is the "report as spam", which takes so long to suspend an account I wouldn't be surprised if the process was manual.
Maybe they can't. They sure lack some core competence by not being able to block obvious spam accounts (Newly created accounts that: posts trend spam, do mass following and mass @mention, etc).