I highly doubt there's any regulation for it. Most of the time it's done because that's how gas bikes are (except for scooters that don't have a clutch, because a foot brake isn't as easy to implement on them). Even the ones that don't have a clutch like Honda's DCT bikes still have a foot brake. Also being able to brake with the foot is useful, for example if you're stopped on an incline but need to use your hands for something else or just want to keep your brake light on at a traffic light or junction. If I ever get around to fitting a hand rear brake to my bike I'd like to add a splitter so I can keep the foot lever as well.