Okay, will explain this as plain as I can in simple terms.
Free port. Pirate inside. Pirate pops out, pops in before invincibility timer runs out. Pops out again. People outside the port are getting annoyed. Pirate pops in port before the timer runs out. Boats outside disperse, because there's no way to engage the pirate who keeps on docking the port repeatedly for protection. Pirate pops out, nobody outside, pirate goes for a nearby player trader. Harassment.
Is this explanation simple enough now?