So you would see no problem in feeding people in homeless shelters bowls of boiled sawdust? There is no responsibility to feed people properly when you accept the responsibility to feed them?
What other things do you think you can do improperly after you've promised to do them?
In the context of this discussion, you said that McDonald's is "accepting responsibility to feed people" by offering free lunches.
If you opened a homeless shelter, and tried to feed people sawdust, that would not be morally wrong. It would be stupid, because nobody would eat it, but it wouldn't be wrong. There is no responsibility for you to "feed people properly" (whatever that means) just because you offered to feed them something.
Do you really not remember that this entire discussion is about McD giving free lunches to people who qualify for free lunches?
No, this discussion stemmed from a comment that if McDonald's offered free lunches, other restaurants would have to do the same or shut down; and I pointed out that we already have evidence that people are willing to pay a price premium for quality. Whether or not people who cannot afford to eat at McDonald's today would start eating the free lunches is really not relevant to that discussion.