That's the rule for every kind of reservation, not just DVC. It is MUCH easier for recordkeeping purposes to have everyone on the reservation on the plan than to have two on the plan, three not on the plan, etc.
If you want, you can add the guests who don't want DDP to the room after you arrive, and they won't have DDP. The biggest problem with doing this is if you need Magical Express transportation from the airport for these guests, in which case they have to be listed on the reservation ahead of time.
When the DDP was new, DVC guests couldn't get it. It's a package add-on and was not even available with DVC reservations. After the DVC members protested, it was fixed so DVC members can purchase the DDP. That's what is meant by "DVC doesn't have to allow members to purchase DDP" although I think that if the removed the ability to do so there would be a huge outcry (from those of us who don't now consider DDP too expensive anyway)