MY help is NOT allowed to bring ANY food in my home and I make sure that she has whatever she likes to eat available. She eats very well in my home.
Keeping that in mind, KOSHER establishments should NOT allow ANY outside food whether it be by customers or staff. It will serve them better and cost less in the long run for them to supply lunch and break food themselves instead of taking these chances.
As far as D. Donuts are concerned, FYI, they serve other food aside from doughnuts.