If the restaurant wants to obtain their desired food cost 'quota' for that month, the prices will change due to the increase of cost. Lets just go with a sauce, beurr blanc, contains a high amount of fat (butter). That essentially will increase a lot in price just due to taxes. Now the food cost determines quite a lot for a restaurant. The food quality, the pay per hour, the salary, etc. It covers for maintenance, for replacement equipment, utensils, utilities (water, electricity, gas, etc). A portion is also given to investors (if any). Now unless you live in places where the expense is generally higher (I'm looking at you Las Vegas, being one of the biggest offender) then the restaurant needs to compensate for the increase in price.
Getting a profit margin at even 5%+ is usually harder in this economy, in fact rather hard. It may average around 2-3%. Though some restaurants may be able to get around 9-10%, it is within the minority.