pypilot is "free" as in freedom.
Freedom is especially important for an autopilot, because without it there is no way to prove someone could not remotely take control of your boat underway and alter course. There is no way to prove that software could cause the pilot to stop working after a certain amount of time, and there is no way to inspect, or analyze the program for problems.
Freedom to modify the program means everyone in the world can collaborate on improvements large and small, and all users potentially benefit from them.