I think the issue is more that they're polluting the social network and the quality of links. I'd bet that most of the "followers" that these bots attract are

I think the issue is more that they're polluting the social network and the quality of links. I'd bet that most of the "followers" that these bots attract are other bots that auto-follow anyone who follows them. Nevertheless, that increases the number of pages with a link to their site, from a high page-ranked site, and thus they profit. That's obnoxious and rude, to say the least. This kind of noise isn't nearly as offensive to the service-user as email spam (for exactly the reasons you mentioned). However, Twitter could potentially be an interesting data set for study and experimentation, and these schmucks are souring the integrity of that data. Link-spamming is basically just a special case of this kind of general data-abuse.