Difficulty in policing something isn't a reason to allow a crime to be legal.
The jury has been out on that for a few hundred years now. Consider 1.) An unenforceable (de jure) law ultimately rewards the dishonest while punishing only the honest who confess their crime (Hobbes). And 2.) From a political standpoint, unenforceable legislation creates the appearance of real moral authority without risk of alienating those constituents who would be punished if were enforceable. Public disregard for enforceability therefore promotes dishonesty of both the citizen and the official -- exactly the sort of business an ethical society should avoid.
Two is not equal to three, even for large values of two.