I think because GR had boasted to the K/S writers in advance that he had added a little something just for them. So they took the term -- and Kirk's footnote -- as open to interpretation.
Well, at the time, simply saying "gay couples exist and are accepted in the 23rd century even if Kirk and Spock aren't one of them" was a major step forward. It's certainly more of a nod toward inclusion than Rick Berman was ever willing to permit.