Why do black labs cross their paws?

They are just resting. This is not really a breed trait, many dogs do this, but you will see it often on dogs, like labs, who have longer legs in proportion to their bodies (also called "up on leg"). For example, the Borzoi (Russian Wolfhound) dogs are more often seen with their front paws crossed than out straight. It is probably the most comfortable position for them when on the floor.