If a service simply pulled a list of tweets from users that you might be interested in there's nothing wrong with that, correct? It's just curating a list of tweets for you. There's also nothing stopping you from favoriting them all to get the most attention from those users.

But when the human act is entirely faked it's spam. And I agree. But it's also a pretty slight distinction we're making since humans can easily choose to behave like bots.