But aren't police unions partisan in the US? Is a police officer standing outside a polling station representing their union's politics? Must feel so to some people.
Maybe, but at some point, every single human is partisan. Police officers aren't affiliated with a party, officially, so they are at least semi-neutral. And since local police live in the same community, they're incentivized to be civil to their neighbors. Who would want the scandal of doing something nefarious in their own town's polls?