I don't really want to knock anyone, either, and volunteering is great, but I find the implication that there's something skeevy about paying a fair wage to the people who supply a charity's backbone labor to be kind of bothersome.
Subtextually, there seems to be a peculiar double standard being presented: Paying retail prices for in-kind donations, which implicitly covers the wages of all sorts of retail and distribution workers associated with the grocery store, as well as lining the pockets of the grocer store's owners and, if it's a public company, shareholders, is just fine. But providing cash donations is bad, because then the charity might use some (presumably less) of that money to pay its own employees to do those jobs? And paying them money is bad, because they should be getting all that labor for free?
It's hard for me not to see that as a tacit allegation that one's time and labor are inherently less valuable when they're being given to an organization whose mission is to help others instead of one whose mission is to make its owners as rich as possible. Which, personally, I think that my opinion runs close to being the exact opposite of that.
If you want to spend your free time helping others, that's awesome. If you want to make a career of helping others so that you can spend even more time doing it, hey, that's awesome, too. And, the world's distributions of wealth, free time and skills being what they are, supplying the money that enables people to go that route can quite easily be the most effective option still other people have for giving back, so that's awesome, too.