Humans live by a series of commonly accepted behaviours, norms, and laws. Someone who abides by pirate law, lives their life by their own set of rules which may or may not be accepted by the rest of society. Under this philosophy, one takes what they want without concern over consequences or compassion for others. To follow such a philosophy is to live and behave much like a pirate.
If a person steals from others - not because of need but out of want, they are living by pirate law.
If a person stops or interrupts traffic to cross the street - not because they are in a rush, but simply because they want to cross, they are living by pirate law.