Article I Section 2 Paragraph 3 of the constitution:
"Representatives and direct Taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers, which shall be determined by adding to the whole Number of free Persons, including those bound to Service for a Term of Years, and excluding Indians not taxed, three fifths of all other Persons. The actual Enumeration shall be made within three Years after the first Meeting of the Congress of the United States, and within every subsequent Term of ten Years, in such Manner as they shall by Law direct."
This allows for the government to take a decennial survey to 'enumerate' the population for the purpose of appointing representatives. It doesn't matter what USC that congress passes that says otherwise because nowhere in the constitution does it give congress the authority to create such a law. I suspect this is why no one has ever actually been fined, because if the fine was challenged in court on a constitutional basis it would most likely die a horrible death. Than there would be legal precedent for refusing to fill out these intrusive ACS forms. Of course the Census Bureau doesn't want that.
My opinion, I am not a lawyer so take it for what its worth (not much), is simply ignore it. If you get Census Bureau employees on at your front door just ask them to leave and/or trespass them from the property, but DON'T lie to them. Lying to census employees can indeed lead to a fine, can't seem to find the cite that I used to have for someone being fined for lying on the form. Can anyone verify? And certainly don't threaten them, they're still federal employees not matter how obtuse they can be.
If anyone has a different interpretation of the law, please let me know. I'm always happy to learn more.