[Platapus]

Generally, in social circumstances, you should recognize the intent behind the act, and not focus on the act itself.

When the host puts food on your plate, they are not doing it in a way intended to make you feel bad. They are putting on your plate what they feel is an appropriate amount of food.

Graciously accept it and eat what you want.

In many cultures, America being only one of them, having a guest not having "enough" food is embarrassing. So the host makes sure that each guest has plenty of food.

From the host's viewpoint, it is better to offer more food than the guest can/wants to eat then to offer the guest not enough food. This means that frequently the host errs on the side of offering more food. This is not an uncommon custom in many countries.

In America, it is not considered an insult to the host to leave food on your plate as long as you are lavish with your complements. It is a win-win consummation.

However, if you eat all the food on your plate, the host may worry that you did not get enough to eat. This is why in many cultures it is polite to leave a little bit of food on your plate.

The take-away is that the host is doing something with the best of intentions. They don't mean to make you feel bad, in fact, they wish to facilitate you doing your part of the custom -- eat and leave some food.

Is it wasteful? Well yes. But in a social environment, criticizing the host for wasting food is not going to make the event easier.

Judge a social act in the light of the intent of the person doing the act, not how you interpret the act itself.