OK, so DU use to make a commission (from Disney) off of people who booked through them because they didn't know how to do it themselves or wanted DU to keep an eye out for a better deal for them. Now they will still be making that commission off of people who don't know how to do it themselves, plus they will be making $64 ($84 after introductory) off of people who normally do, do it themselves. Like you said, DU & the customer both win!
If someone books through DU, not DISavers, DU gets the commission.