Growler Fills

I have been a craft beer lover for about 20 years, and have collected quite a few growlers. Some for display, some for filling.
Before taking to fill, I always make sure they are "beer clean".
The majority of places I go to have no problem filling up my growler with another logo. However, Moody Brews will not do this and they say it's against regulation.
Ok, I'll buy a Moody Brews growler.
But I don't understand why I should have to? Can someone explain this to me?