Strawman Has a Point: Madame Natasha is paid to teach students ballet, which includes teaching them how to follow designated choreography. Kristen's improvisations could look like mistakes to a conservative or traditional ballet critic. Changing the steps every time would be frustrating for a director, not to mention difficult for other dancers to adapt to when she isn't dancing solo. This is doubly true for aspiring professional dancers, who often spend several years in the corps de ballet after joining a company, where it is crucial for a dancer to be completely in tune with everyone else on the stage. Improvisation in the corps will throw everyone else off.