I am not sure if this business decision by Marco was made for the same reason I am speculating but I am doing it anyway - they recently opened the door for free users(invite only) and some usage limit which IMHO is the limit most of the paying users would be practically exercising unintentionally, because there just not many people to interact with. So, they just needed more users and instead of opening the sluice gate they made a crack in it; of course lowering the cost was another crack!

Besides, everybody who is on App.net is on Twitter/Fb and posts whatever they post on either sns.