I agree that it might be complicated to legislate.
On the one hand, you might just use an IP address and allow hotel/rental/remote watching for finite amounts of time (say 30 days), before you assume that the viewer was actually "resident" at this non-"primary" address. But I easily think of instances of people who work on the road pretty much all the time, or perhaps spend weekdays in one location and weekends in another, and so on. They might try to do this on the basis of different users on an account not using the service simultaneously in multiple locations. But I can easily think of circumstances where one person works remotely for periods of time away from other household members, perhaps for weeks or months at a time. Are they going to be forced into multiple Netflix accounts?
And those "extra member" fees aren't super-cheap either.
(And why is there a €2 difference between Portugal and Spain? I guess because the Spanish catalogue is much better than Portuguese language one...)