I noticed that in U.S. airports, there is a charge of around $5 to use the luggage carts. Generally, the only place I see them free are in international arrivals; in departures and domestic arrivals, one has to pay. (After searching this site a little bit, this also appears to be the case for Canada.)
However, in many other airports, usually outside the U.S., luggage carts are provided free of charge, in all airport zones.
Why is this?
Given the answer to this question, I don't think this is too broad; the answer is mostly the same nationwide because of a federal law that restricts the fees airports can charge passengers as part of their airfare, to just $4.50 (as opposed to, say, Heathrow's nearly $52)