I saw Willie at the '76 "Rolling Thunder Review" in Houston, and recently my step-daughter saw him in LA (quite a span of popularity for legendary Willie).

That said, elements of your argument are kind of like a "its not fair that cattle can't be admitted to the horse hall of fame". Exceptions made for the transcendent ones I guess (ie Bob Dylan in Rock/Folk, Elvis in Blues/Rock HOFs etc)

I recently took a "History of Jazz" class, and was kind of frustrated by the vague definitions of Jazz (though most of us recognize the art form, which typically involves improve, syncopation, certain instruments, etc) I couldn't figure out how it included Ragtime and Glen Miller swing (I was stuck on a stricter Ken Burns definition). It seems "rock" and "pop" are conflated (the way my parents looked at 60s music), yet they should be distinctly different. To add, many musicians cover a wide range of music styles.

Look, maybe Willie is great enough to be triple threat, being in country, rock, folk HOFs (kinda like RIta Moreno winning Oscar, Emmy, Grammy, Tony). He should be the exception though, for fear of diluting the accomplishments of other (strictly) Rock musicians.