maybe saying that they are not musicians is a little incorrect...
Some of them deserve a little more credit than that, a few of those can actually sing, less of them have written their own music and none of them really play an instrument other than sing so there is a bit of a drop off.. but musicians all the same
Although elvis and the beatles were pop stars before they became 'musical geniuses' too so maybe we have to wait and see? (almost swallowed my tongue comparing them to beatles so don't hate on the comparison too much!)