That is probably the only reason
DVC members have discounts like 10% off merchandise and 15% off tours. I highly doubt that a 10% across-the-board merchandise discount yields greater profitability for Disney.
10% isn't much but it is better than nothing. Disney makes huge profits on their merchandise trust me. I know how much a lot of this stuff costs. My company regularly gets pricing on products similar to what they sell and the profit margin is through the roof.
Fortunately (for Disney), these types of events can only accommodate a few dozen--or few hundred--guests at a time. Meanwhile the four theme parks + 30K hotel rooms host tens-of-thousands of guests every single day. It only takes a very small cross-section of their patrons to regularly fill these special events to capacity.