The legislature branch makes laws, the executive branch executes them, by passing regulations that enforce them, often to the degree that the executive sees fit. This is constitutional. That Obama saw fit to show compassion was his perogative, as it was Trump's to pardon Joe Arpaio. Oh - and the judicial branch interprets laws and how they are applied; Arpaio was convicted of ignoring the judge who informed him to follow the law as it was. Thus Arpaio committed an illegal act; Trump's pardon does not change this fact.