John Lennon is looked at by society as the ultimate revolutionary and leftist. However, I think he was more confused and easily manipulated politically more than anything else. He didn’t start writing all of these protest songs and speaking this way until after he met Yoko. I think she was the revolutionary. He had nice things to say about Reagan according to his assistant.
http://www.dailymail.co.uk/news/article-2009562/John-Lennon-closet-conservative-fan-Reagan.html
Did he just change his mind? Who knows, I think it was just more political naiveté and confusion amongst other things. In the end, as much as I love John Lennon’s music, if you read about John Lennon the man, he was not someone to be admired.
Could also be the drugs, and the perceived “coolness” of being being part of that radical movement, such as our screwed-up culture has been since around that time.