Open source was the rule rather than a choice in those days. Nobody would pay for source code that only ran on your hardware. It was only in the 80's that the closed source systems started to emerge.
"Nobody would pay for source code that only ran on your hardware."
Unless your hardware was a mainframe. IBM distributed quite a lot of code in source form, with the implied assurance that there were no cheap, alternative means to run it. OS/360 wasn't even copyrighted, which is why it's legal to run on today's PCs. IBM, without trying, and probably even without realizing it, was an early pioneer in the open source niche of this "Pay-It-Forward" culture.
"SHARE Inc. is a volunteer-run user group for IBM mainframe computers that was founded in 1955 by Los Angeles-area IBM 701 users. A major resource of SHARE from the beginning was the SHARE library. Originally, IBM's operating systems were distributed in source form and it was common for systems programmers to make small local additions or modifications and to exchange them with other users. The SHARE library and the process of distributed development it fostered was one of the major origins of open source software."
