Which laws don't we enforce and why?

Tim Wu, a smart and funny law prof, has a new series up on Slate that he calls his magnum opus -- a series of articles examining which laws America doesn't enforce, and why:
This series explores the black spots in American law: areas in which our laws are routinely and regularly broken and where the law enforcement response is ... nothing. These are the areas where, for one reason or another, we've decided to tolerate lawbreaking and let a law--duly enacted and still on the books--lay fallow or near dead.

Why are there dead zones in U.S. law? The answer goes beyond the simple expense of enforcement but betrays a deeper, underlying logic. Tolerated lawbreaking is almost always a response to a political failure--the inability of our political institutions to adapt to social change or reach a rational compromise that reflects the interests of the nation and all concerned parties. That's why the American statutes are full of laws that no one wants to see fully enforced--or even enforced at all.

Link (Thanks, Tim!)