Cops are not lawyers and are not a valid source for interpretation of laws. If they were, then no one would ever be found not guilty.
"The presence of the sign makes it a contract that you were in violation of."
Please (I'm not being sarcastic)address the part about "This note is legal tender for all debts public and private."
How can the presence of a sign in contradiction to that not invalidate any presumed contract to begin with?