"Either (a) Twitter is terribly bad at detecting spam or (b) there's something about Twitter that makes detecting spam difficult or (c) they don't care.

Based on my experience detecting spam, I'd guess (c)."

"Twitter engineering: If you're going to do such a bad job of catching spam, how about at least giving us a one-click way to report a tweet as spam and block the account, like email providers do? It may even help you get better at filtering, since more reports = more signal."