That the restaurant could have posted such a policy would seem obvious. That it did not is a conclusion based upon the fact that the manager said "Our understanding is a buffet is just a style of eating," rather than something like, "As we note on our sign, we reserve the right to limit servings."
Perhaps, and perhaps not. No one should assume a news article is going to be truthful, full, complete, or impartial.
But even so, in human contract law there's a term they use, 'a meeting of the minds.' Assuming neither side was out to defraud the other, there are instances when justice declares that each side failed to meet the standards of a contractual agreement. Making a public scene to the point of eviction by Constitutional officers is the antics of freeloaders, blackmailers, and extortionists. But we all know how evil those businessmen are and how they deserve to be smacked down and taken advantage of if anyone artful enough can get away with it, eh?