Twitter recently updated their Twitter API Frequently Asked Questions to include an entry explaining their decision to disallow bulk following and bulk unfollowing.
"Why are automated and bulk following/unfollowing services not allowed?
First, it’s worth clarifying that this policy change isn’t meant to prevent developers from building tools that surface potential accounts to follow or unfollow. However, the follow/unfollow action must happen one account at a time, not in bulk.
After hearing feedback from users who rely on third-party tools for follow management, we want to provide more context behind this policy change. Per theTwitter Rules, following or unfollowing large amounts of users in a short time period is considered to be a spam activity. Bulk and automated...