It was mainly an industry initiative, for whom the label "progressive rock" resonated nicely with the radical rhetoric of the era. CBS promoted some of its rock acts as "The Revolutionaries," and there was their famous "The Man Can't Bust Our Music" ad campaign. Warner released a compilation LP called "The People's Album," its cover art and lettering leaving little doubt as to its political intent:I am not quite sure about your political association with the term progressive rock. I have been a fan of the various subgenres now associated with the type of music now known as progressive rock, years before the term had been codified as it is used now. But I never remember this political association being mentioned.
Critics in the rock press were increasingly linking the music to the countercultural politics of the moment.
Yes, I recall the same, and increasingly commonly beginning in roughly late '66 [e.g., Cream]. The first Caravan LP [late '68] is often cited as the first use of the term specifically in relation to the music that would ultimately assume the label "Progressive rock," what we now call "Prog."I distinctly remember reading, quite a few years ago, that the first print use of the term 'progressive rock' to describe a band, was in the late 60's English music press, and it was used to describe Cream, John Mayall and some others. I can't remember the exact source for this, but it seemed pretty legit.