Legislative Clarity Quotes
One of the reasons that judges and government officials have much difficulty in enforcing and interpreting laws is that the laws themselves are often very vague and/or ambiguous. Often this is the result of a political compromise or one part of a law being at odds with another written be a different legislators. Also, this is a way of Congress avoiding tough issues—every member can say what it is that he/she voted for and the vagueness allows their opponents to claim that it means just the opposite. The Executive and Judicial Branches then have to resolve the issue.
See Learned Hand {628969} for a later expression of this notion.
He was specifically referring to the Marijuana Laws where some prosecutors enforce the laws but most do not.