Why cant we say BA BA BLACK SHEEP??? racist?

personally i think this is bizaarrrr!! i could understand if people were using it in a derogatory way. in the way that people say p*ki because that is quite rightly deemed as an offensive word. BUT ba ba black sheep!?!?!? really?!?!? ALSO we are not allowed to say Brainstorm?!?! because its offensive to eppileptics, we must say thought shower. ANNDD students are now LEARNERS apparently??