^I think the Americans started that, Nishin.
In France--as I understand it--a chef's creation is often the result of many years of hard work. When a customer tries to change that food item, he's implying that the chef doesn't know what he's doing, and so the chef gets profoundly insulted. I've heard that if you insult a French chef, they'll sometimes throw you out of their restaurant.
Somehow' on this side of the pond, we got it backwards. We got this idea that "the customer is always right", so many customers will pick apart an item on the menu, substituting this for that, etc. The chefs over here get insulted, too, but that's not where the problem lies.
The trouble that the customer doesn't get a consistent product. Let's provide an example.
The customer wants a ham-and-egg sandwich. It's not on the menu, but the waiter takes the order anyway. Cook #1 cooks it the way he thinks it ought to be cooked. The customer loves it.
The customer comes back a second day, and gets a different cook. Invariably, the second cook's idea of how to make a ham-and-egg sandwich will differ from the first cook, so the customer will get a different product.
This causes the customer to complain. "Why didn't I get it like I got it yesterday?" So the manager has to refund his money, or throw away the first sandwich, and cook him another.
Sooner or later, the Sous Chef finds out about this, and gets enraged. "Well, why did you let him order something that wasn't on the fuckin' menu in the first fuckin' place?"
I have watched this scenario repeat itself possibly a thousand times.
Moral: order what's on the menu, and take it the way it comes.
Exception: if you have a food allergy, we will work with you. Tell us if you're allergic to onions, for example; often, we can leave it out without a problem.