This behavior can be explained in terms of marginal costs and marginal benefits.  At a standard restaurant items are priced individually - they have a positive marginal cost.  If you order more it will cost you more.  You order until the marginal benefit from the extra food no longer exceeds the marginal cost.  At a buffet you pay a flat fee no matter how much you eat.   Once the fee is paid additional food items have a zero marginal cost.  You therefore continue to eat until your marginal benefit becomes zero.

Economic theory consists of factually supported generalizations about economic behavior. Economists use two methods to obtain sound theory: induction and deduction. In deduction the economist starts with a hypothesis and tests it for accuracy by gathering and examining facts. In induction the economist starts by gathering facts and then examines their relationships so as to extract a cause and effect pattern: a theory. Regardless of how derived economic theory enables policymakers to formulate economic policy relevant to real-world goals and problems.

As for the quotation the opposite is true; any theory not supported by facts is not a good theory. Good economic theory is empirically grounded; it is based on facts and so is highly practical.

(a) (d) and (f) are macro; (b) (c) and (e) are micro.

(a) and (c) are positive; (b) and (d) are normative.