Do you think the landlord should be allowed to make them remove the sign?
If it's accurate, what's the harm in the public knowing about it? If it's a two square meters stand in a food court, the public might find the landlord to be insane. If it's a five thousand square meter super market, I'm sure the public will find that appropriate.
Keeping the Apple tax secret suggests that Apple believes the public wouldn't find them appropriate, either because "the public doesn't understand" (easy to fix by writing a blog post) or because they aren't appropriate (given Apple's profit margins, I lean towards this).