This YouTuber is a huge guitar nerd and I just love this video for the way it shows how much he engaged with the equipment in the various parts of this part of this store(? or is it a convention?) before he tries playing Stairway to heaven, and how fast he is shut down each time. Also love his response to the shut downs! 
What is the reason for the shut downs?
Here’s a video on that topic. I don’t know if it is actually as universal as these videos and other media make it out to be, but I know it as a trope.
It’s a joke. I’m sure he must have set it up in advance with the vendors, in return for highlighting their merchandise.
Yes, staff at a convention or guitar store might roll their eyes behind your back, but in my experience, you can Stairway, Smoke on the Water, or Sweet Home Alabama to your heart’s content, as long as it leads to a sale.
I figured the staff might be in on the joke for this video, but I didn’t want to clutter it with my complete outsider’s suspicions. 
I’d like to see a youtube rebuttal with you just playing Stairway to your heart’s content in store after store. This is a widespread belief that needs to be challenged with solid evidence.
I think Wayne’s World started it. Just my experience (a few decades of it), but I have heard all kinds of playing in guitar stores—superb, horrendous, hackneyed, whatever—and invariably the staff is enthralled, pointing out that the playing is evidence that the person really needs to buy whatever is in their hands.
I don’t know if it existed in the real world but the legend definitely pre-existed Wayne’s World. I had heard of the list of songs you weren’t allowed to play in the '70s. Maybe the legend only existed to teach young guitar players some manners.
In 1978 a salesman played Stairway for me in a music store on a Yamaha FG355 I bought and still own. Also “Long Train Runnin’” (don’t know if that’s on the list). He also sold me a book of Led Zeppelin tabs. He could just see it in my face.
If I tried that in a guitar store and was shut down I’d say “but…I was playing the opening to ‘Taurus’ by Spirit.”