They don't own the words "open source" no matter how much they might like to.

> “Open Source” describes a subset of free software that is made available under a copyright license approved by the Open Source Initiative as conforming with the Open Source Definition.

No it doesn't. It describes software whose source is "open" which is generally understood to mean that you can read, modify and reuse the code for free.

Public domain definitely fits that. The "public domain doesn't exist in some countries" arguments are spurious as far as I can tell.