My guess is they are pooling points by using a points / miles broker. The broker offers to pay them money to pool their points with several others he is buying points from, along with the person who is buying points from him. He profits from the sale of the points, and the person buying from him gets a pretty good deal. However, this violates the T&C.

This is a common issue in the AA forum - people get their accounts closed for selling miles to a broker (or buying from one). I'm guessing either Hilton didn't have the means to detect this previously, or that pooling points has made it significantly easier.