The mare is hobbled for her own protection and the stallion's. Even if the stallion is not injured if she kicks out at him, he could become mentally unwilling to breed since he now associates mounting with pain.
Breeding hobbles do not prevent the mare from moving from out from underneath the stallion. On the contrary, she can move forward relativley easily, the hobbles just prevent the hocks from going past the point of the buttock- prevents her from kicking out. I have seen mares get spooked by the hobbles quite a few times, but when they are adjusted correctly, they will not cause her to get tangled. I highly recommend using a chain link and leather breeding hobble since these will not wrap around legs as easily as a nylon one would. So in other words, breeding hobbles do not enable humans to allow stallions to "rape" mares.
Hobbling a mare for breeding with regular hobbles also has its place with a maiden mare that has been determined to be in through teasing, palpation or ultrasound, but is either scared or doesnt like the particular stallion. Some mares, even if not maiden mares, will prefer experienced stallions over unexperienced. If an untrained stallion starts stumbling all over the place, she may decide she's had enough. I also think that to avoid scaring the wits out of her, application of the hobbles should begin long before a stallion starts to try to mount her. That experience in itself could be quite scary.
If any of this is the case, and other than TBs, instead of risking the safety of the mare and stallion, or potientially "raping" the mare, it is very easy to have her artificially inseminated these days--especially if the owner of the mare is very concerned about rape...