Traditionally, spam has comprised unsolicited advertisements such as the ones for V1@gra you find in your email inbox every day, or links published in the comment sections of blogs. In short, spam has always been unwanted information pushed by the spammer to the spamee.
Twitter has always made it difficult for unwanted information to be pushed to a user -- one must choose to follow another user in order to receive updates from that user. But spam on Twitter has come to mean not just the provision of unwanted information, but the consumption of information as well. Among other things, Twitter defines spam thusly:
Commercial or promotional use of Twitter is allowed. There are many companies who create valuable, opt-in relationships with users on Twitter who want to keep up to date with them. However, if you are following other accounts in order to gain attention to your account or links therein, you may be considered spam. [emphasis in original]Take a moment to consider what this means: potentially accusing of malfeasance users who are requesting to view what is for all intents and purposes a public stream of information (viewable on the Twitter public timeline and even indexed on Google). It's not unlike me accusing you of a malicious act for subscribing to this blog. Furthermore, the language is so broad as to be almost absurd; if you don't want to gain attention for one's Twitter account, why would you undertake the essentially exhibitionist act of opening one to begin with?
Now, I'll confess that I've a vested interest in all this, since I've undertaken activity on Twitter that would probably be considered spamming under this definition. I opened an account for my band, and started following a bunch of people I didn't know (for the record, hardly anyone I know in meatspace is even on Twitter) who lived in Chicago and/or expressed interest in alt.country music. Admittedly, part of my motivation for doing so was to alert those I was following to the existence of my band; however, I also follow their Tweets to find out what's going on in the city where I live and in a musical genre that I love. How will Twitter decide whether that action constitutes spam or not? By way of an algorithm? Or by means of the human spam wranglers they have begun to hire?
Of course Twitter users should have the right to keep their Tweets out of the hands of those they don't wish to see them. But Twitter already makes that very easy by providing the ability to make one's updates secure, requiring approval of all followers. Defining any act of following a Twitter feed as potential spam will only hamstring the social networking potential of Twitter, as users become afraid to reach out to people who don't already know them. Instituting algorithms to decide the intentions of human beings will only lead to a lot of unhappy users.