We reserve the right at all times (but will not have an obligation) to remove or refuse to distribute any Content on the Services, to suspend or terminate users, and to reclaim usernames without liability to you.
Which, if I read this correctly, allows Twitter to suspend an account for whatever reason.
http://support.twitter.com/articles/18311-the-twitter-rules
Though, the TOS has a leeway:
We reserve the right at all times (but will not have an obligation) to remove or refuse to distribute any Content on the Services, to suspend or terminate users, and to reclaim usernames without liability to you.
Which, if I read this correctly, allows Twitter to suspend an account for whatever reason.