Last Saturday, I was walking around downtown Chicago (at the corner of Michigan and Wacker) when I came across an unusual sight: A homeless woman begging for money right next to a young woman with an open pizza box who was giving pizza to people who passed by. To me, the most unusual aspect of the situation was that the homeless woman was not eating some of the pizza.
The young woman was advertising the pizza place around the corner (Bacino's). Setting aside the fact that there was a beggar nearby, it is an interesting advertising ploy to give some of your product away to potential future customers. Doing so can provide information about your product to people who wouldn't know about your product, which is one important reason advertising can be effective.
Viewed from this perspective, it is a rational (but heartless) explanation that the young woman would simply say no to the beggar. The beggar was clearly without means to be a potential customer at this restaurant (at least in the near term). From a business standpoint, it wouldn't make sense to waste advertising expenditure on someone for whom there's no pay off.
That caveat aside, I can't imagine that the young woman would say no to the beggar and, as nearly as I could tell, the young woman was giving the pizza away indiscriminately. In my mind, the reason must be that the beggar didn't ask for pizza. If the homeless woman was hungry, why wouldn't she grab a plate? Maybe she was too embarrassed to ask for food (but not money?) or maybe she didn't want food. Then again, if she wanted food, maybe she preferred McDonalds.