It's not just ginormous SUVs with this problem, though, right? You're not going to see a 18 month old out the back window of your compact hatchback if they're too close to your car. Especially now that windows seem to be tinier than they used to.
Right, backup cameras make sense even for sedans and other small cars. The high-hood trucks and SUVs in the US are the reason we'll probably get mandatory front cameras eventually as well.
No, it's common to all vehicles. You can't see small children behind a small passenger car, either.
Blaming trucks and SUVs for everything is a favorite pasttime of internet comments, but all vehicles benefit from backup cameras and collision detection sensors.