As long as there are tables for people who want one, they won't say anything. I think it's more about the people who come in at 10:30 for lunch and sit there for hours with their buddies sucking down free refills, when people who want a table can't find one. So now when they want to kick these people out, they can point to the sign to justify why they are doing it.
There's a McDs nearby which I occasionally stop at. Sometimes at 5:30AM, sometimes at 9AM - yet the same people are always there. But it's not crowded inside for breakfast, so nobody minds.