It has to do with the
DDP causing folks to feel compelled to order food whether they are hungry or not, just because they have credits to use up .... then don't eat the food because they aren't hungry or not their tasting. We have talked for years here about how wasteful the DDP plan actually is, and sadly when you add folks who are rude and litter ... which this is, it's not pretty.
When people pay outright cash for meals they tend to be more reserved in their ordering and make more of an effort to actually eat it.