Why did The Peanuts make the black kid sit by himself at Thanksgiving?

I stumbled on this still from "A Charlie Brown Thanksgiving." The clip embedded below shows the scene. Fast forward to two minutes in. Sure enough, Charlie Brown and his pals make Franklin sit by himself:

Photo: "Retarded Children Benefit Show"

The photographer says, "I found this while looking through the window of an old movie theater. The place has been closed for at least 25 years." Via.