Ok here is the story directly from the Director's mouth.
They put the Disney characters in and then asked for permission, they showed the movie to Disney and Disney was "
sweet to them" and let it stay or they would have had to cut the scene. Disney also realized it was good promotion. All Bob Clark's comments.
He also commented that the Characters stayed in character and was not too far out of line.
So I guess you could call this Product Placement.
Also Jean Shepherd himself is seen in the scene as the man who tells them that the line to see Santa ends here, it begins there.