Patrons don't just walk onto the grounds, whether having a ticket or buying one at the gate. It's strictly controlled. A limited number of tickets are made available through a random drawing. However, most have been made available in the past through waiting lists, which have been closed for some time.
For a short example on how things are run, here's an example. Scroll down to the "Prohibited Items List" for an idea. They don't fool around, and this guy had it coming to him.
I don’t see where acoustic guitars, bongo drums, or even bagpipes are banned. Seems you could have Burning Man there.