If you order something in a restaurant and you don't like it, why should the mgr. take it off of your bill?

to keep people from causing a scene or badmouthing their restaurant. i work at a place that's not corporate and they dont' usually do it as much as a regular place would because they know that the customer is going to tip off the reduced price rather than the full price of their meal. we had one lady that ordered the same thing 3 times and tried to get it taken off. the first two times it worked, but the 3rd time both managers were there and recognized her immediately. some people are just out to get something for free and the big corporate places have lower food costs and can afford it easier than they can afford bad reviews that would affect all their restaurants.