Despite all her fame and glory, Rihanna should have behaved professionally for her fans, who are the reason she is successful. She should live up to her responsibilities as a superstar. She should have arrived on time. A short delay to build anticipation is acceptable, but a wilful disregard of her fans is not.
All stars should consider that they are in the public eye and that many of their fans view them as role models. Their fans' perception of them can change if they act badly, and this can affect their future career success. For example, Rihanna's unprofessionalism makes me less likely to buy her music.
Lateness is a small issue compared to other things that stars may do. Some stars lose their reputation - or even their lives - due to drugs or alcohol.
Celebrities should always consider their actions.