General-purpose operating systems evolved from the mire, but for a very long time the source code was still available because the vendor didn't make any money selling you the code, they made money selling you the hardware.
That very fact is the key to open-source as a business methodology. It goes back to basic economics. Any product in the marketplace naturally has other products made by other companies that are used in tandem. A car requires gasoline to run. Portable radios require batteries. Computers require software. Such products are said to be compliments of one another. Increased sales of a product implies an increased demand for