I don't think it's poorly worded at all.
First, the point of abilities like Assassinate is to give the PC a critical hit when they typically wouldn't. That includes Magic Missile.
Second, given the ruckus over Magic Missile in 4e's Essentials, I think the designers of the game, and the writers of the PHB, knew how special the term 'hit' was and wrote the entry appropriately. I think they knew what they were doing when they used the word 'hit' in the spell description.
Third, 2d4+1 per missile on those occasions when it's applicable isn't exactly game braking.
To be poorly worded the outcome would not have to be unintentional, but game breaking.
Poor wording doesn't have to have game breaking implications.