You are actually charged an extra 10% fee for sitting down in the restaurant. If, after DIY ordering, carrying your own food you don't sit down, but pay and exit you save 10%. It's kind of strange that way. Often people who do choose to sit leave extra money at the table, on top of the automatic sit down charge of 10% on their bill.
I think dining plan folks get out of the 10%, that's kind of nice for them.