This film sets up a strawman and absurd premise. In fact Clockwork was also banned in Easter European socialist and communist countries as well. Banned in Poland, Hungary, USSR, Bulgaria, Romania. Also banned in Japan, Ireland, Singapore.
It was NOT the general nihilism or a political impetration of that which cased the bans, it was a) the celebratory graphic rape of the Dutch girl while the rapists sung "Singing in the Rain" and b) several direct copycat crimes copying the "ultra violence" down the actual assult in the film and the costumes.
This is a time when LIBERAL societies as well as conservatives blamed, rightly or wrongly, severe increasing graphic violence in film, TV and other entertainment for violence in the real world. In the US, DEMOCRAT Al Gore and his wife made "offensive" and "violent" lyrics in songs, especially African American Rap songs their target, calling for bans.
This documentary is laughably in a bubble, making this about Franco and the church of Spain when it was a worldwide phenomena.
Burgress (the author of the novel "clockwork) and Kubrick (the film adaptor) delayed or reduced release in the UK itself due to some sensational copycat crimes.