Why do police enforce laws that are unjust?

I am sick of hearing police saying that they are just following public policy by arresting non violent offenders. How is that even a justification? The nazis public policy during Hitler's reign was to kill and imprison the jews. So if you asked a nazi and he said "well I was just following orders", how is that even remotely a justification. I have a lot of family members that were ex law enforcement. Including aunts, uncles, and my grandfather. They told me that they would never arrest someone for a minor offense. Such as small quanities of drugs (various drugs not just marijuana), public intox or anything that is non violent or benign. Dealers would get arrested however. My uncle would never issue traffic citations because he knew that was not actually helping others, it was just getting the state revenue. He said that the police frowned upon the fact that he didn't make his quota, but he still managed to become a high ranking detective. He told me people who speed are going to speed no matter how many tickets they are issued. He also said he couldn't bring himself to bring someone to jail for having illegal drugs in small quantities. He said that speeding is way more dangerous than the drugs themselves, so how could he justify bringing the drug users to jail when speeding is way more harmful. My grandfather was the same way, he preached to me that he went after the rapists, the murders, the arsonists and the child molesters. He called drug users "small potatoes" and that he went after the dealers themselves. Crack, Amphetamine and PCP were the drugs that my family members said were the worst. Usually if caught with drugs like that, they would be jailed. So my question is, if my family members carried out a halfway decent policy, why can't other members of law enforcement do the same?