The Weak US Government

One of my hobbyhorses--how really weak the Federal government is. I get support, albeit unknowingly, from a surprising source--a libertarian. Ilya Somin at Volokh Conspiracy writes:

My parents and I were green card holders from 1979 to 1986. As far as I know, they rarely if ever carried proof of legal residency with them except when entering and leaving the country. I suspect that most other legal immigrants behave the same way. Why? Because the chance of running into a federal law enforcement officer in everyday life is infinitesmally [sic] small.

This is in the context of a discussion of the Arizona immigration law.  Somin is worried because people have or could have dealings with local and state law enforcement officials almost every day.  (In fact, I've had very few interactions with such officials in the course of a rather long life.  Maybe I drive slower than Somin.)