The rotors are rusting away because the brakes are rarely used on these cars. Regenerative braking is regularly enough to slow down these cars during normal driving, with brake pads only being used sporadically. I routinely see Priuses go 150-200K miles on original brakes.
How thick are the brake pads in the picture? I see substantial brake pad material remaining there. My opinion, if the brake pads were being replaced the rotors should have been as well. But I would not have recommended brake replacement yet since the pads aren’t worn to 3mm or less.