Trivia: Lindsay Lohan was originally cast as Regina George. She thought the role would be different than her previous "damaged teenager" characters. However, according to the casting director, Paramount wanted a "bankable" actress for the starring role. Lohan was switched with Rachel McAdams, who had been considered for the role of Cady.
Trivia: Tina Fey, the woman who plays Ms. Norbury, was also the screenplay writer for the movie.
Trivia: Tim Meadows, who plays Mr. Duvall, broke his wrist a week before filming started and had to wear a cast. They explained this away in the movie by saying that Mr. Duvall had carpal tunnel syndrome.
Trivia: Amy Poehler, who plays Mrs. George, is only seven years older than her on-screen daughter Rachel McAdams, who plays Regina.
Trivia: Lindsay Lohan originally wanted to read for the role of Regina.
Trivia: An interesting fact that I was able to confirm was purposeful by watching the commentary on the DVD- Tren Pak, the popular Asian girl who makes out with Coach Carr, is the on-screen sister of T. Pak, an Asian nerd on the Mathletes (same Asian nerd who says Regina's hair is insured for $10,000.).
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.