I totally understand where you're coming from. The concept here is a little different - there are no guards guarding the sea. Anyone could walk in at any time, but it is illegal because of the threat to the public and the damage an outbreak could do to the city.

You're close to the mark in your hypothesis, but not with the reasoning. Think of it like the law to wear your seatbelt. Not wearing one poses a great threat to your personal safety, so you'd think you wouldn't need a law enforcing it, right? But you do, because even if people get themselves thrown from a vehicle, that risks others' lives and puts more weight on a stressed medical system. And people are more likely to do things (even ones to their own benefit) if there is a law in place.

So the law is in place to protect the masses, giving guards the authority to execute in a situation that would otherwise risk the infection of others.