Actually has been explained by yoshi the reason of this, and sound reasonable: PVP.
He made an example where a melee engage a mage : the mage cast a powerful nuke (let's say thundaga), the melee run away to avoid it, and the mage follow him without interrupting the cast = melee dead. Since would be painful introducing this feature where pvp will be live, they are releasing now so we can stat praticing.
It's actually a really valid argument in a game with pvp. I'll miss casting in movement, but i'll live with it