Please tell me the mechanism by which a company learns the demographics of their customers - this information is not collected at the point of sale.
There are a number of ways to get demographic data from a customer or customer groups. Your online presence and activities (sites you visit, product you purchase, key words that you type, etc. can give data mining companies more information about you than you might think. These company collect and correlate various sources of the data to come up with a "picture" of people like you. They don't need to personally identify you to do this. This is big business, and companies like WoTC can purchase that data.
Demographic data can even be surmised with brick and mortar purchases as well. Especially if you sign up for "membership", for example at Barnes & Noble, or your local gaming store.