People are so daft about stuff like this. I don't mean you personally Kerry but even so!
Firstly, people who are homeless aren't walking around in a permanent state of starvation! There are other more pressing needs on the street: keeping warm, keeping dry. Keeping comfortable. McDonald's offers this.
Secondly, you know when sometimes you fancy a certain food just because? Homeless people do too. And you're limited in the types of food you can have - think about it, it isn't just not having a cooker or microwave but no plates, cultlery, bowls etc. Even when DH (then DP) and I lived in squalor we could have a bowl of cereal. So on the street, what "filling" foods are available? Not bloody many. Pies, pasties, sandwiches, burgers, chips, crisps, chocolate, fruit
if you go to Aldi but that's not very filling either (I always feel hungrier after fruit, except bananas, but I digress.)
Thirdly, I'm not homeless, but I get lonely and isolated during the day. I like to sit in Starbucks or caffe Nero with dd and watch the world go by. Being homeless is isolating and lonely.
There are three excellent reasons why, if you're homeless, McDonald's is a GREAT choice. Cheap. Warm. Hot. Social. Even comparatively healthy!