ENFORCEMENT FIRST. Tyler Cowen makes a valuable point in the immigration debate, noting that the seemingly commonsense principle that "at least we should enforce the law" is actually a bit odd. Lots of laws are very imperfectly enforced and this isn't necessarily a problem. The police deliberately downplay the level of resources they dedicate to solving burglaries in order to focus on the more serious crime of murder. But they don't completely cease trying to arrest, prosecute, and jail burglars just because not every murder gets solved. And as everyone knows, lots of people get away with speeding on highways, but many other people get tickets, and the modest amount of effort put into enforcing the speeding laws has a real impact on behavior. Nobody, however, seriously thinks we should hire enough state troopers -- or create a federal Interstate Speed Limit Enforcement Agency with enough manpower -- to perfectly enforce the speed limits.
With immigration, as with everything else, half-hearted enforcement has real effects and it's not in any way unusual for something to be illegal while, in practice, lots of people are able to get away with it.