Principalmente perché Jimmy Page è un Thelemita dichiarato, possedeva una libreria occulta e vive nella casa di Alister Crowley.

It was mostly just a bunch of bitter Christian parents that disapproved of any form of secular music. One of the reasons why was also due to the seemingly overnight fame that Led Zeppelin attained. It was also a very popular groupie rumor that Jimmy Page, Robert Plant, and John Bonham sold their souls to the devil for fame, girls, and money, but John Paul Jones refused to do so and that's why he's the lesser known member of the band and that none of the horrible things that happened to the other guys happened to him, but that's a story for another day. People also bought into the rumor because of Jimmy Page's fascination with the occult, black magick, and Aleister Crowley.

So to recap, Led Zeppelin were considered devil-worshippers because:

• Bitter Christian parents.
• The insane overnight fame of Led Zeppelin
• … of a groupie rumor that said that Jimmy Page, Robert Plant, and John Bonham sold their souls *rolls eyes*
• Jimmy Page's fascination with the occult, black magick, and Aleister Crowley.