[themafia]

I believe they were both "accidentally open" for similar reasons. Neither company produced the most important chips and components in the device itself. That meant that you could assemble a greater understanding of the device than even the manufacturer had and there was good incentive for putting this effort in the early days of computing.

[musicale]

Intentionally open. As noted above, Apple published schematics and ROM source code. IBM published system board schematics as well as the BIOS source code.