Visible crew/equipment: Before Cady enters the house, when one of her best friends is watching a movie the reflection of a crew member is visible on the TV.
Visible crew/equipment: When Damien and Janis drive off leaving Cady on the sidewalk with a picture, a large square stage light on the set is reflected in the window of the back car door (probably one which lights the ground to give the effect of a streetlight). It is too large and bright to be one of the lights from a house across the street.
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.