It's because of the bad press on things from FFXI like 18 hour Pandemonium Warden fights.

SE got heat in the media for outrageous time commitments in their MMO, so they designed a system in their new game to force either horizontal progression (if you want to play 8 hours a day) or make people play less.

Experience tells them that even if they have warning saying "don't play too much!" that people don't listen and they have to mother their players. It's a foolish think to think, but it is what it is.