The main reason "Stairway to Heaven" is banned or considered "forbidden" in guitar stores is largely a humorous cultural joke rather than a strict rule. The song is famously overplayed by beginners trying out guitars, and store employees get tired of hearing the same iconic riff repeatedly. This phenomenon was popularized by the movie "Wayne's World," which featured a comedic scene about banning the song in guitar stores. So, the "ban" is mostly in jest and a nod to guitar store etiquette to avoid playing the overdone riff that many people attempt because it is easy and famous. Additionally, the "forbidden riff" nickname has some other lore such as alleged satanic backward messages in the song, and the fact that even Led Zeppelin's singer, Robert Plant, reportedly grew tired of hearing it and once paid a radio station to stop playing it. In summary, the "ban" is a mix of the song's iconic status, its ease of play leading to overuse in stores, cultural jokes from films, and some supernatural myths, making it a humorous tradition rather than an official ban.