If TVs have to display multiples of 24 FPS (those TVs that can accept a 24 FPS signal) to avoid flicker, why do not movies in the theater flicker at 24 FPS?

Because most movies in theaters are projected at 48fps with each source film frame being displayed twice with a shutter closing between the repeats (essentially producing a quick black frame your eyes aren't fast enough to see) which helps to reduce the perception of flicker. Remember, film is projected by shining a light through it and this is different from the way any TV works and your eyes perceive projected film differently.

A few people do see flicker in movie theaters though.

It has nothing to do with flicker. They are trying to avoid an uneven cadence on pans when the "adjustments" have to be made to convert 24fps to 30fps.