The whole point here is that it's a mis-use of a messaging channel - they are leveraging the fact that users _don't_ expect favoriting to be a form of advertising, they expect it to be a message from an actual human being that has favorited their post because they like it. (Similar to the problems with follower spam.)
(I'd also point out that ads on web sites at least help subsidize user's free usage of those web sites, so there is an indirect benefit to the user that isn't there in the case of spam.)
You may have your Twitter notices already turned off - I get emails from them when someone favorites or replies to my tweets, as well as notifications from their app.