Barnstorming is a term referring to the golden days of flight where stunt pilots would use bi-planes in search of performing their aerial act. They would often land in a corn field with an amazed farmer staring in disbelief and wondering what the crazy contraption was that brought this man from the sky. The pilot would then negotiate with the farmer for an impromptu show for the locals to enjoy.
Barnstorm busking (or gorilla busking) is the same attitude of adventure. The busker simply sets out for a pitch and performs their show without regard to any restrictions the city may have. The busker reasons that their show is short and should the police arrive, they'll be finished and long gone.
The busker can hardly concern himself with City Hall constantly changing the laws concerning busking. He's too busy making a living, traveling the world, and meeting new and interesting people.
One busker had just arrived in Zurich on a Friday and the airline had lost his gear. In the meantime, he talked with the locals about busking. The first thing they told him was it wasn't allowed on Sunday. He had spent all of his money waiting for his gear to arrive and needed to busk. Sunday or not, he took a train into town and set up in the main square on the Neiderdorf in front of God and everybody. He sang loud and proud for ninety minutes without a care in the world and raked in the most money he'd ever made. The busker never saw anything that would lead him to believe it was not legal on Sunday. He honestly couldn't have cared less about any silly laws. From then on, he was one of the most successful buskers in Switzerland, legal or not.
"It's easier to beg forgiveness than ask permission" speaks volumes of the differing attitudes concerning busking and street performing. If you're constantly asking permission, you're bound to run into the default answer of "No!" much more than if you hadn't asked. I seriously doubt any polite busker has been arrested and jailed for a first-time offense of busking. The police would rather "move you on" because they really don't want to fill out forms in triplicate for such a minor offense.
However, if you plan to stay in town for a while, it's probably best to avoid no busking signs or call the police beforehand to see what you can expect.
You can always pass the hat to make bail...