Interestingly, Wikipedia's entry claims (without citation!) that .org "was originally intended for non-profit organizations or organizations of a non-commercial character that did not meet the requirements for other gTLDs." [0]

I believe RFC 920 [1] was the guiding document when .org was established in 1985. It cites no restrictions on .org second level registrations, merely that "ORG = Organization, any other domains meeting the second level requirements."

[0] https://en.wikipedia.org/wiki/.org#History

[1] https://tools.ietf.org/html/rfc920