This. Linux wasn't based on proprietary code, and wasn't saddled by any sort of license. The original BSD that Bell Labs gave to Berkeley in 1974 was anything

This. Linux wasn't based on proprietary code, and wasn't saddled by any sort of license. The original BSD that Bell Labs gave to Berkeley in 1974 was anything but open source. It was an operating system that Bell had provided to the university with source code available for modification and redistribution ONLY to other universities WHO ALSO HELD research licenses from Bell Labs. And it still wasn't open-source in the early 90's when Berkeley scrubbed all the proprietary bits out of it and tried to sell it for profit. That's when AT&T sued them. So even if you HAD a copy of the BSD source code pre-1994 it wouldn't have been the same as someone handing you code and asking for your advice on it. You would have certainly found yourself in court if you modified early BSD for redistribution.