So, barring statewide laws, it's devolved upon the counties to make there own rules regarding this. Calvert's requires the cleaning and surgeons general warning labels, and allows places to refuse to fill ones that are not theirs, and does have the restriction that they must be from in-state breweries. Which is stupid, very few places, if they mark them at all, mark them with what freaking state they are from. My Craft Brew Exchange one from South Carolina doesn't mention that it's from there. My Bavarian Barbarian one from a one man brewery in PA that's since shut down doesn't. My two from Charlottesville, ones marked Devils Backbone, one unmarked at all, but neither gives a state My two Ruddy Duck ones don't mention they are from Maryland. Do they keep a up to date list of all breweries in Maryland, of course not. St Mary's I don't think they have actually adopted one yet. Which means each place can do what they wish.
All reports I have says that ABC only fills their own. Part of the profit center. They will use the excuse that since they use a machine that pressure fills vice a bartender, I think that's BS. The machine is designed to fill a standard 64oz glass growler. And it contains the growler inside a protective case.