It is really hard for me to understand this line of reasoning. I lived in Canada as a foreign student a few years ago and it was way cheaper to cook for yourself than to eat at a McDonald. Maybe the situation is different in US, but I doubt it.
McDonald's isn't cheaper than cooking for yourself, but time constraints (consider a single parent with multiple jobs) and/or the up-front costs when bits of your cooking infrastructure is lost, broken, damaged may make the marginal cost of getting the next meal from McDonald's cheaper than the cost of repair plus the next meal. Given that the poor may have limited access to additional credit and litte-no reserves, that marginal cost difference may make McDonald's the only affordable choice, even though the long-term cost is higher.