Vaccines are a public health issue. In order for the population to benefit from vaccination, almost everyone has to be vaccinated to create herd immunity. If a neighborhood vaccination rate is too low, then the youngest people and the immunologically compromised have no protection against diseases that can kill them.
When parents choose not to vaccinate their children, they are not just risking their own children's health and life, they are endangering other people's children. And those other people are probably not very happy with the prospect that someone's stupid decision risks *their* children.
If those "other people's" children are vaccinated then what is the problem?