Hi! Long time lurker first time poster here. I linked to this from a recent article of Scott's, so I'm a little late to the party.
A couple of things struck me about this: one, people who support an organization's work or 'want to help' may be (in many cases justifiably) suspicious about the way money is spent. Giving labour gives them some control over how that labour will be used; if it's used in a way that is obviously counter to the stated aims of the organization/what the individual volunteering actually cares about, they can leave.
Secondly, people may care enough to give their leisure time but not their working time. It would be absolutely correct to say that this is less of an indication of caring then giving money, but its a reality that people will be more willing to part with their labour (money being a representation of that labour) if it's done in a leisurely way, ie spending a morning volunteering at a soup kitchen chatting, rather than working alone in an office.