This is not a sermon, but an answer to a question that needs a spiritual answer.
God is Spirit. Usually when one gets a limb amputated, it is because that limb has become life threatening. So the miracle is to remove it and the body becomes healed or is no longer threatened with death from a diseased limb. The bible says And if thy hand offend thee, cut it off: it is better for thee to enter into life maimed, than having two hands to go into hell, into the fire that never shall be quenched; And if thy foot offend thee, cut it off: it is better for thee to enter halt into life, than having two feet to be cast into hell, into the fire that never shall be quenched; And if thine eye offend thee, pluck it out: it is better for thee to enter into the kingdom of God with one eye, than having two eyes to be cast into hell fire:
God could have said he would grow these limbs back, but he said to cut them off. In the natural, he wants us to cut off sin that is in our lives. Amputees are his angels that remind us that we need to cut off sin in our lives. God desires that we prosper and be in health even as our soul prospers. Amputees are left with limbs that are alive and non life threatening. It is the will of God to cut off sin and this just symbolizes in the natural world a spiritual principle. You say why did God cause these people to suffer? Their reward is in heaven. He did not cause them to suffer, but he did not say he would grow limbs back, he said he would cut them off. It is his will and God can do anything he pleases. There is more to it than this earthly life as we know it. Jesus gave more than a limb, he gave his life so that we may be saved and forgiven for our sins.
For every one shall be salted with fire, and every sacrifice shall be salted with salt. This says there everyone will be tested with fire. We do not live in this world without being tested. We all have burdens to bear. Some people have limbs cut off, others have other burdens. But this explains that the amputee’s fire is having a limb cut off. Symbolizing how we are cut off from God when we sin. That is why he won’t “heal” (I presume you mean grow the limb back). I hope this explains it to you. Bottom line is God wants us to be cut off from sin.