It's not like using Twitter doesn't come with costs; you need to learn how to use it, you need to follow people to stay up to date, you should probably reply to some of the people tweeting at you; and last but not least, you also need to spend time on actually coming up with things to say. That's a massive time investment. The opportunity cost of that time is a lot more than the $5 app.net used to cost per month.
I assume that $5 a month is only a problem for occasional users -- but I doubt that those add a lot of value to the ecosystem (aside from higher numbers of total users signed up and other vanity metrics)