Rock and Roll as a category has become much more elastic in the popular mind than in mine. As with many of you, my definition starts with 50's and 60's rock and roll and then moves to the artists whose music carried a strong connection to that music.
Artists who were a little outside of that in soul and R&B were soul and R&B artists (not necessarily rock), but not letting them into the tent would seem elitist. For example, Sam and Dave were more soul than rock, but who in their right mind would want to exclude music that was so objectively great and influential (and had such a great beat)?
But from there, we have a sliding scale. Michael Jackson was the King of Pop who only dabbled in rock (and quite well with "Billy Jean" and "Dirty Diana"), so he wasn't a rock artist, but he was too popular to leave out of the tent.
So then we get to including Hip-Hop artists such as Run-DMC, who included guitar in some songs largely to add appeal for a white audience. NWA had a rebellious attitude that aligned with rock's image, but what else did they have in common with Elvis or The Beatles or anyone else in the "Rock" area of the record store?
I used to be upset that certain artists were overlooked while others that were more "pop" than "rock" were included, but then I learned to stop caring. I have my record collection and my own hall of fame, and I enjoy it immensely. And after I'm gone, it won't matter anyway.