As an unrelated aside, it's also a weird phrase (though absolutely common in usage!) as the privileges of the nobles and peers, for example, were way more difficult for the crown to abrogate than were the ordinary rights of mere commoners. Privileges were strong. Maybe being able to employ, and deploy, your own soldiers would have some effect on that.

It would probably be better if the phrase were "a grant not a right" or "a license not a right", but I guess that's not as catchy.