I know of one group that this rule was put in place for and its the group that has their dj system set up right in front of the yellow parking lot gate as your walking towards the stadium. Its blaring loud and after the games, their drunks azz are singing karoke. I will be happy if they shut down that group all together.
I guarantee you that very few, if any, have ever been annoyed by Texan Mojo's music. Our's isn't that loud to begin with, although our speakers are certainly larger than 12" x 18". Besides, no one makes anyone park right next to us and if you do park next to us, you almost certainly know what you're in for.
Those dipshits near the gate, on the other hand, like to crank it up so loud that's annoying even 100 yards away. And as mentioned above, they like to turn their speakers outward from their party so it sounds louder to folks hundreds of feet away than it even does for their own guests.
I like the spirit of the rule. Let's hope they don't go overbard with enforcing it.