I've had to drive an occasional windowless van for moving stuff. And I've packed my car solid so there is no rear view. It is definitely off putting when you glance up and there's no headlights behind you but you know you are surrounded by vehicles.
But if it's legal to have moving vans with no rear window, how would you justify that it's illegal to block a rear window? You can literally take the passenger version of a full-size van, and the cargo version which has no rear windows, block off the rear windows on the passenger van and suddenly it's illegal.