It’s not quite like natural, natural is a word that would predate any comoany trying to take it for their own ends. “Open Source” as a terminology was coined in 1998 by a group of people, and the OSI was formed by one of those people that same year to help define what the term would mean and encourage industry adoption. An entity that has existed as long as the term has and helped shape it’s perception to programmers is a bit different than an entity coopting a term that had prior use.

Regardless, I’ve always understood Open Source to also mean open to use, extension, and repackaging myself. The exact definition indeed doesn’t have to be owned by the OSI, but their guideline are a pretty good approximation of what I would expect code to do to be truly open source.