That's not the point. Anyone has the right to sing if he/she likes it. But from the moment you put the video online, you accept to be criticized.
I sometimes happen to sing. But I know I'm a terrible singer so I keep my singing for my bathroom. I would never sing in front of a webcam and then put the video on youtube.
This girl didn't wanted to sing. She wanted the world to kno how she sings. Now the world knows. And they have right to express their opinion about her singing.