I've seen people allowed on buses with food, I've also seen the drivers tell them no way. A lot depends on what it is. Water bottles, never a problem. Crackers for kids, not usually a problem. An ice cream cone or cotton candy, no way.
I think it really depends on the time of day, type of food, and how crowded the bus is.
Food in a sealed container is always OK, as long as you aren't eating it on the bus.
Anne