He's not a spammer, and the service actually benefits the tweet authors to some degree which cannot be said for spam. Favorite counts are posted publicly, which at least theoretically adds to the user's credibility. Additionally, the accounts doing the favoriting will have some real followers, and those followers will see the favorite and might retweet or favorite it themselves.

Given these facts, I'm not sure how many Twitter users actually care if the favorites they receive are genuine or not. The reality is that few users on Twitter are real anyway. We once paid thousands of dollars for a tweet to go out to millions of followers of a celebrity. The cost wound up being about $207 per collected email address for a free newsletter that was highly targeted to that celebrity's followers. We got tons of hits to the link, but approximately 94% of the traffic to the link posted was bot traffic. This was a celebrity that the ad network we used to purchase the tweet had rated as having extremely high follower engagement metrics - apparently they didn't bother checking user agents when determining this. If you want actual users listening to you, don't go to Twitter.