Because cops work for the government, which has a monopoly on policing services and the tax revenue that pays for it. The unions can therefore be far more abusi

Because cops work for the government, which has a monopoly on policing services and the tax revenue that pays for it. The unions can therefore be far more abusive, as their patron and customer, which is the taxpayer and resident, respectively, is captive, absent these two groups being able to successfully organize a strong enough political coalition to counter the Democrats in the electoral process.