Food isn't as fungible as change, but there's still opportunity cost. Each dollar they don't have to spend on food is a dollar that can be spent on drugs instead.
True, but it's not like they're eating at expensive restaurants. Anyone can get a full stomach with $2 or $3 at a McDonald's or Taco Bell. And just an hour of begging on a major pedestrian area will get you enough for that, a pack of smokes, and a bottle of whisky.