It does mention "a well-regulated militia" though, and oddly legal gun owners don't belong to one of those.

The point about militia is a short statement for why the right to bear arms is important, rather than trying to restrict or qualify the right.

The ability to form militias is so important, that everyone should have the right to bear arms, in order to enable this.

The idea is that it prevents the idea of a "special militia" having some selection criteria, so the government of the day cant make qualifying for its group a requirement to own guns.