Restaurant industry in the U.S. as a whole has determined that, generally, large parties undertip. Everybody thinks somebody else got the tip, or when they split the bill either people undercalculate the tip or just don't take it into account, etc. Mandatory service charge listed on each - or even just one - menu page, with its conditions, is designed to avoid this problem.
Just as when one orders a $7 appetizer or a $16 entree or a $2 soda or a market-price lobster, one expects to have pay the amount listed on the menu or quoted by the server, parties consisting of the indicated number of diners or more should reasonably expect to have to pay the posted percentage as the service charge.