That's also a GREAT point. Many of these accounts aren't even real. A bot can leave comments as well, so a lot of these follow for follow people are just bots. It's best to stay away from them.
In fact, since I've written this post, at least 3 more "follow for follow" users have followed me. Becuase of these people you can't trust your growth numbers as having any value on this site right now.