By putting someone "in your family" you at least feel like you're giving them access to a lot of stuff, so you probably don't do it with friends as much. Plus there is some unique tracking by account even if it's just an email address. I never tried it but I would assume I can't be in multiple "families" at the same time on Prime or iStuff.
Whereas if you're just Netflix, and the only reason I use you is to stream Netflix, what is the barrier to handing out my password?