Or they might not have the money to spare. Not all economic decisions are constrained by preference -- there is a real point in life, occupied by a great many people even in the US, where the desire for a McDonald's Cheeseburger is basically irrelevant to the fact that right now, you cannot afford it.

You don't seem to be using the word "preference" in the same way I am (or economists do).

If I have only $2, and so can't afford a $1 cheeseburger, then we say that I prefer $1 to a cheeseburger. You have a choice--either keep your $1, or get a cheeseburger--and you choose to keep your dollar. This seems perfectly clear.

In the extreme case, when everything you own and your labor is literally worth less than a cheeseburger, then this doesn't exactly work: it may be that you would rather get a cheeseburger than a $1, so that you will buy a cheeseburger at literally the first opportunity. Very few people are this poor, and those who are don't generally buy hamburgers as soon as they get $1. There are much better ways to spend a dollar: cheeseburgers aren't worth $1 to the rational poor.

Now whether we interpret this as meaning that the poor care less about a cheeseburger, or as saying that they care more about $1, seems to me to be a question of semantics which no one cares about.