I think they deserved to get kicked out. If the restaurant never made the claim that it was all-you-can-eat, they don't have to live up to it.
Code:
Buffet (wordweb):
1. A piece of furniture that stands at the side of a dining room; has shelves and drawers
2. A meal set out on a buffet at which guests help themselves
3. Usually inexpensive bar
Code:
Buffet (dictionary.com):
1. A large sideboard with drawers and cupboards.
A. A counter or table from which meals or refreshments are served.
B.A restaurant having such a counter.
2. A meal at which guests serve themselves from various dishes displayed on a table or sideboard.
Nothing in there about an all-you-can-eat rule.