In most cases police is seen as serving the government and having all sorts of protections from sovereign immunity (confirmed in a case when a SWAT team partly destroyed the wrong house), qualified immunity (invented by SCOTUS, there is no such law) and practical legal immunity based on the fact that DAs rarely prosecute and even juries rarely find them guilty when prosecuted. This is why the ignorance of the law is accepted in so many cases as a defense for police, but not for citizens.