The brewery near where I live is pretty small, so when I mentioned being able to drink there after getting the growler filled, really it's just up to whoever is working at the time I suppose. When you are on a 1st name basis with people who work there, and see them same people at the local YMCA or other places, this stuff can fly.
I can see why it shouldn't be allowed though.
I think they also fill other growlers, but it's like a 1 dollar difference, so 1 dollar cheaper if you use one you bought there. Makes sense.
They usually charge 2-3X more for beer served in the bar vs. the beer sold in the growler. Like every beer/drink served in a bar, it's customary to have a huge mark-up for the opportunity to hit on the bartender and other patrons or just get away from the wife and kids for a couple of hours.
I can understand a lot bartenders not caring or even some owners not minding the regulars that spend a lot of money there doing it, but as a regular practice, it would be silly for bars to mark down beer served in growlers only to let people drink them in the bar.