What do you mean by thrown for real? (Not just being argumentative - I think a back roll is easy and natural at fast speed - so maybe you and I aren't talking about the same thing.)
I'm not trying to be argumentative either; it's difficult to communicate about physicality fully in this medium.
If I am throwing into a backwards fall correctly, I will have locked my partners spine and cause their feet to suddenly thrust forward (as if stepping upon a roller skate). Either one foot or both feet. The fall that results will not be a back roll.
The only time that back rolls seem to work in technique is when uke chooses to go before the terminal point in their posture where their spine is still not locked and they can still take a step forwards or backwards.
In our system, if you can move like this as uke, you should not fall, you should take sente and reverse the technique, unless you're in kata training and you then wait for tori to figure out how to lock your spine and pin your feet to the ground or eventually recover to show tori that they missed the window for the technique depending upon where they are in their learning process.
Does that help explicate what I mean?