Because they aren't flying a flag of the country in which their boat is registered. All boats do this. Not doing this is like flying a plane without filing a flight plan.

Technically when you don't fly a flag you are considered a pirate. Clearly they aren't pirates but they are smugglers. There is no other reason why you wouldn't fly a specific national flag. You can complain about the rules of engagement and that is fine. However, posts like yours aren't exactly rooted in honesty, competence or good faith.