Yes it is normal, you can shift into neutral but you shouldn't be able to go into reverse without pushing the button. Shifting to neutral while moving won't hurt anything, you just won't be driving the wheels anymore. One reason they allow this is that if the engine dies you can push it into neutral and try to start it again, the starter won't engage in drive but it will in neutral. They make you use the button to go into reverse because it can seriously damage the drive train if you push it past neutral and into reverse while you are rolling forward. I'm sure there are other reasons as well.