Factual error: The mall that the plastics state they are going to, Old Orchard as stated by Cady, is an outside mall. However, the mall they go to is an indoor mall.
Factual error: While the plastics are at the mall there is a Bell Mobility booth seen behind them. This booth wouldn't be found in an American mall as they only service Canada.
Answer: Regina enjoys manipulating people, and she views Cady as a relatively easy person to manipulate, finding her more useful/amusing to keep in their circle. It's a running theme of the film that the friendships between all of the Plastics is very tenuous, because of the catty, manipulative nature of the girls.
Purple_Girl
Also, if Regina tried to ban Cady, people might think she was threatened by Cady. Better to keep Cady around and attack her in subtle, manipulative ways.