They actually can protect your hearing, but only in certain situations, and not in others.

They work by producing sound waves that “cancel out” or destructively interfere with the sound from outside. This is very good for steady background noises like a lawnmower or airplane. But really bad for changing or sudden noises like a gunshot loud concert. In the later scenarios the sound generated to cancel out the outside noise is on a slight delay so it might be too late and it might not be able to fully cancel the outside sound.