ADRs have nothing to do with how you're paying for the meal. I think the primary reason they ask if you have DDP is that many visitors (especially first timers) don't even know there IS a DDP. By the same token, when you call to order the DDP, they will ask if you need ADRs. Many people don't know it's possible to make ADRs, or that it's becoming increasingly necessary to make them (last year during free dining, the problem became so severe they had to come up with an exchange plan for people with meal credits they couldn't use because they didn't know to make ADRs.).
Both of those situations are just Disney trying to make sure their customers understand and take advantage of opportunities.
In addition, Disney doesn't miss many sales opportunities, and they may be able to switch someone from offsite to onsite with the enticement of the DDP. Don't forget that DDP is not primarily a F&B promotion -- it's mostly a promotion designed to entice people to stay onsite.