They use OU-D instead of DE figuring its better if people mistakenly think it is full fledged dairy instead of just made on dairy equipment, instead of confusing so many people.
Similarly, they think it’s better if people mistakenly think something actually dairy is pareve rather than know it is dairy and be confused.
See http://www.theyeshivaworld.com/coffeeroom/topic/ou-kashrus-is-not-reliable