Pedaling backwards just causes the brakes to engage mechanically. If you are coasting downhill, you don't need to pedal, do you? If the motor is pushing you, you are (as far as the rear axle is concerned) coasting, also.
When you brake, the downward force due to weight shifts from the rear wheel to the front. Since the amount of braking action depends upon the amount of downwards force, the rear wheel is very easy to lock up when you're near 20 mph (or more.) This also means that, when you're at speed, front wheel braking is MUCH more effective than rear wheel braking. (You can control the amount of braking much more precisely with your hands than with your feet, also.)
It's important to have BOTH front and rear brakes on a motorized bike, since you have the potential to be traveling at a much faster average speed than when pedeling. If you have a front brake, a coaster brake is OK on the rear, but a rim brake or disk brake is better. (Better control, less likely to fade than a coaster brake.)
Remember, the amount of energy that the brakes have to get ride of as heat, is proportional the the square of the speed you are at; the brakes on a bike at 20 MPH has to get rid of 4 times the energy as one traveling at 10 MPH, all other things being equal, one traveling at 30 MPH has 9 times the energy as one at 10 MPH, and one traveling at 40 MPH has 16 times the energy...