Goes way back to how MS stole code from CP/M.
My understanding was it was Tim Paterson of Seattle Computer Products who borrowed certain parts of CP/M for 86-DOS which was famously acquired for $75K by Microsoft and renamed MS-DOS.
But it's not really right to say it was stolen
since it was generally understood (back in the early 80s) that software was neither patentable nor able to be copyrighted since code was considered merely a set of instructions
Programmers freely "borrowed" what they could. Sometimes with
the original creator's consent. And sometimes (ok...mostly
) without. About your only options - if you didn't want pieces of your product reworked into somebody else's - was to obscure your code, lock up your source, and attempt to copy protect your distribution media.
Short of direct theft by breaking into a vault, or resorting to some form of corporate espionage, pretty much anything else was considered fair play. Reverse engineering. Cloning the "look & feel." Cross-compiling. Porting to a new piece of hardware. All was fair in love an software back then.
It's one reason why so much progress (and money) was made so quickly. A broken patent system, complete with its IP Troll entourage, wouldn't rear its ugly head till many years later.