They probably could, they just value their leisure time more. As I pointed out, many posting on this site have a minimum opportunity cost of $2-4 an hour - by working on Amazon Turk. Why not? Because Amazon Turk is miserable, $2 is peanuts, and they obviously get more utility out of watching a movie instead or something.

I'm not going to judge them too harshly for choosing leisure over work that pays that little, even if it were for charity - I donate very little and I am very selfish and make similar choices all the time, so it'd be hypocritical.

But don't try to tell me that there is no opportunity cost there.