It's a variation on the prisoners dilemma.

The lowest risk option for the herd is that everyone get vaccinated. The lowest risk option for healthy young individuals may or may not be.

The parent is publicly admitting that they chose the more selfish of the two options. This rubs the rest of us (who considered the greater good more important than the incredibly small personal risk) the wrong way.

It's sometimes ok to be selfish, but it's never going to be the most popular decision and don't expect the herd you are a part of to respect your disdain for the well being of the larger group.