IBM simply followed the tradition created by the companies already in the microcomputer market. Apple's documentation, for example, included the full schematics

IBM simply followed the tradition created by the companies already in the microcomputer market. Apple's documentation, for example, included the full schematics and the source listing for the ROMs: http://cini.classiccmp.org/pdf/Apple/Apple%20II%20Reference%20Manual%20-%20Woz.pdf http://cini.classiccmp.org/pdf/Apple/Apple%20II%20Reference%... You could get similar documentation for all Radio Shack products and many others. MITS didn't set out to make their Altair 8800 an "open system". In their case it wouldn't make sense not to have the full documentation for the customers that were building their own machines from kits. But this resulted in clones and in competitors rebranding their "Altair bus" as the S100 (standard 100 pin) bus and making improvements to it as a group resulting in MITS losing control. So IBM should have known what would happen, but probably thought they were too big and important for history to repeat itself.