Apple II was an open system in a sense. Apple published the schematics and ROM source code. But it didn't have well defined interfaces that developers respected. A lot of published software, including some of the most popular apps, made use of variables and entry points in "unofficial" ways. This made it impossible for Apple or anybody else to even know how it was being used, much less to write a compatible ROM or OS that was not an exact copy of the original.

And if an updated system were to break any published app, Apple would be blamed. There were apps, albeit only a few, that would not run on an Apple IIe, and I think, a few more that wouldn't run on a IIc.

There were some notable violations of published entry points in MS-DOS software, most notably the page locations of display memory, leading to the famous "640k barrier." But they weren't enough to dissuade developers from treating the PC as an "open enough" platform.

I doubt that developers felt a particular sense of morality about the DOS interface, that they didn't feel about Apple II, but only that the interface was good enough to use as-is.

The real important thing here, was the openly published interface, and mutual agreement among devs to respect that interface. I mean "open enough" and "mostly respect" of course.

And when people made Apple II clones, most of them (like the Franklin Ace series) got sued out of existence by Apple. Eventually true clean-room ROMs were created like for the Laser 128, but that was fairly late in the life-span of the Apple II.

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.