A friend of mine (who practices freeganism) explained it to me this way:

She is a vegan because, although she has no problem with eating animal products that do not require harming the animal (for instance milk or honey), she does not want to financially support factory farming. She will, however, buy and eat animal products from farmers who guarantee that animals were not harmed or abused in the process of making the product.

When something is free, her eating of the product does not lend any support to industries that abuse animals, so if she can have some cheesecake that will otherwise just go to waste, why not eat the free cheesecake?

Of course, she still doesn't eat meat, free or not. One has to draw the line somewhere.