1 It's a distribution model. Open Source is simply about the source material being openly accessible to the public.
2 FOS (Free and Open Source) is significantly more liberal in it's distribution model. In this case the source is not only open to the public but the public is also free to do as they wish with the source material.
3 If you want to use the opensource.org definition then you're free to do so but words have meaning. The word "Open" in the context of Open Source is analogous to transparency of information.
4 People are not going to read a document when trying to understand what Open Source means to opensource.org advocates. They are simply going to take the words "Open" and "Source" and use deductive problem solving to conjure up a definition for themselves.
5 This is why FOS exists. Free and Open Source cannot be mistaken for anything but source material that is freely accessible, modifiable, usable, and distributable by anyone.