Just because it is called the "Rock and Roll Hall of Fame" does not mean it will not feature other music besides Rock n Roll.
All music tends to have other influences within, in order to become what you hear as that style. You say BB King is cool...do you think his music was not a huge influence on the guys at Motown? What about someone like Robert Johnson, who influenced BB King...Stevie Wonder, Smokey Robinson and all the guys at Motown were influenced by previous people before them- that is a given.
Now, who do you think Stevie, Smokey and other Motown artists like Marvin Gaye, Michael Jackson and The Four Tops, The Commodores and Rick James influenced?
Without Stevie Wonder, certain qualities of the Red Hot Chili Peppers wouldn't be there, nor half of music (especially the entertainment side or performance) in regards to Michael Jackson. Nor would you have certain commercial success' like "Superfreak" Rick James or Lionel Richie (or Nicole!)....
...all paths lead to somewhere. If they lead somewhere, they came from somewhere too. Hence the reason you see Motown on the RnR HofF.