You misrepresented what the article said and left out that they listed cases where people were refused service based on dress.
'No shirt, no shoes, no service' holds. 'No blacks, no muslims, no disabled' would not be legal.
Here's what the sign should read: 'In consideration of all our guests, we ask that parents please monitor their children. And silence your cell phones. No substitutions. And absolutely NO CLOWNS!'