> rapidly cycle their prius between 50 and 70 on the freeway because regenerative brakes save gas (I felt carsick for hours after arriving at my destination)

Weird take to me, unless you were on a lot of hills; at least in my Maverick [0] 55-65 is 'ideal' MPG range for long trips, going between speeds tends to trip things up and actually -avoid- the weird 'battery has enough juice where we just kinda lug the engine' mode.

Doing regenerative 'braking' compared to using physical brakes, absolutely can give energy for momentum/acceleration and save on the physical brakes wear and tear, OTOH any normal cyclist would say it's better to 'maintain' a given output power vs allowing deceleration and then going back up to speed.

As for why, well I'm not a physics person, but in general it's that you are having to overcome the rotational mass/etc of the wheels (i.e. tires, axles, etc), and no regenerative braking within the current laws of physics will make slowing down and speeding back up more efficient, at least on a flat road.

[0] - OK It ain't quite a prius but it works fairly close aside from overall drag...