Not to defend startups, but it's not like they don't have a reputation for paying less than full value for their technical staff either.

If I was a designer and a startup wanted spec work, I'd just politely say no.

Wagging a finger and telling a startup why they shouldn't do so is time better spent on something else, because there's a big assumption that they care. If they did, they wouldn't have asked for freebies in the first place.