Why did the Beatles promote drug use?

The same reason any other bands 'start' - because they like music

Lennon, McCartney, Harrison and Starr were all enthralled by the rock'n'roll movement; inspired by artists like Elvis Presley, Carl Perkins, Buddy Holly and many many more

In their time (the late 1950s was when they first formed) rock'n'roll was seen as a rebellious outlet for young people, and each member of The Beatles had his own reason to rebel