What the businesses I saw this at did was the waitstaff worked the tables part the time and prepped take out orders part the time. So in effect delivery workers were stealing time they could use to wait tables or directly serve a takeout customer that would usually tip them something. It wasn't that they had staff just for takeout that wasn't getting paid regular wage.

Naturally the waitstaff super resented having to maybe take on less tables to do essentially unpaid work on the side of taking your order, prepping utinsels and drinks, possibly even making some very simple ready made stuff themselves, and plate bagging it up etc and a lot of the shit they have to do for a normal table except for no tip.