Intentional failure to enforce the law or regulation is obstruction of justice. In recent memory, several cases come to mind. The Obama justice department failed to charge several individuals who were harassing voters in Philadelphia. The Obama justice department failed to indict Hillary Clinton on bribery charges for selling access to government agencies who could accommodate the individual paying the bribe.
These are political offenses but the predominant ones are those every day occasions when the cops let speeders go, simply because there are so many. The other more common ones are plea bargains where a miscreant pleads guilty to a lesser charge for whatever reason, either the case is weak or some money changes hands. During the prohibition era, the police just didn’t enforce the law because politicians who control the police were being bribed.
The biggest mistake in the Clinton issue was not prosecuting her when Trump came to power. It is probably the reason Commey was fired by Trump because he refused to prosecute. Why would he refuse. He was appointed by Obama and the Democrats and Republicans both, do not eat their young. In other words, the selling of access is a means for politicians to make money from their office and one of the perks of getting elected. It is corruption pure and simple and has gone on since the beginning of time. It has become so commonplace that it is no longer considered an offense.
Trump had to be told that is how the system works although he was probably on the other side in many of his own business dealings except he was at the local level. Incorruptible politicians rarely make it to the top at any level.