I am guessing the reason is twofold. One, they know it is unlikely a family would pay for more than one account while living in the same house, but they might

I am guessing the reason is twofold. One, they know it is unlikely a family would pay for more than one account while living in the same house, but they might get a few extra bucks for the family upgrading to a multiple streams subscription. Two, they probably have data that a shared account in the same house does less simultaneous streaming on average than a shared account in multiple houses.