The point is that Ebony quit because it was too hard.
Had Tyra told her "Oh, it's ok. I understand." That would be worse becuase it would be an indirect way of telling Ebony it's ok to quit whenever things get hard. That type of attitude would get Ebony nowhere outside of the ghetto.
Telling her "Quitters disappoint me" is another way of telling her that whatever she does in life, things aren't always going to go her way so at one point she's going to have to tough it up or continue being mediocre.
It seemed Tyra wanted her to go to the finals and maybe her quitting over something so expected like criticism just upset Tyra because of the potential going down the drain.
I like Ebony, there is something very sweet about her but the girl needs to toughen up if she wants to make anything of herself. Tyra was right in what she did though.