Parents need to teach kids to take care of them selves. This is a age based question, elementary kids need to acknowledge that they're being bullied and report it then parents or the school step in. A middle schooler should be forced to deal with it him self. A high schooler should be able to ignore it.
Suicide is one of the leading causes of death in the U.S. Most suicides are a result of online bullying. If you allow your children to get bullied, then there is a good chance that they could get so upset that it ends in their death. Protecting your child from bullies could ave their life.
Of course people shouldn't sit idly while someone is being harassed. There is a point though when the abused must stand up for themselves. Individual strength is the basis of a healthy community, if someone can stand up for themselves and their own interests there would be a lot more people being invested in other things regarding self interest.
No one should let someone bully, but people should be encouraged to take action.