I think they do this because some of the restaurants require a credit card to book and if they know your on the DDP they dont charge the credit card but if your not on the DDP the charge you at the time of booking. As i said could be wrong.
No, I don't think that they try to sell you the dining plan if you are not on it. I think that they do it so they can note the reservation and then the restaurant server ends up seeing it and then they know you are on the plan.