If they can play their voice and interpret songs etc. they are musicians.

Mel Torme = musician
Eta Jones = musician
Adam Duritz = musician
Feddy Mercury = musician
Patsy Cline = muscian

Britney Spears = shock vocalist (really, have you heard her live? visit youtube)
Justin Bieber = current industry golden hen. soon to be fried chicken.
Beyonce = booty shaker
Don Dokken = screamer and big hair at it's worst
Lady Ga Ga = ??