Police unions are not ordinary ones. Ordinary unions have to fear beatings by the police. Police unions not so much.
They prevent disciplining officers for brutality. They influence elections to elect pro-brutality politicians, including the 2020 election. When citizens protest police brutality, they beat protesters.
Unions can be regulated. Police unions are a breakable link in the chain of police brutality.
Police unions should be forbidden to defend members against discipline. They should be forbidden to endorse candidates or organize politically.