I think its a Korean thing ( I get a message that comes up at 3 hours of game play and says, "You have been playing for three hours, please log off for you health."
As well I think its a tactic to stop gold farmers and bots.
Edit: Alsowhen I prepurchased I think I rember signing up for a monthly sub. $14.95 for 1 month no hour limitation that I remember reading.