Developers do speculative work for clients all the time. So, for that matter, do software security people, and the dollar value of an hour of software security work dwarfs both web devs and designers. But don't feel bad, because lawyers also do work on spec, and their bill rates crush ours.

Designers need to stop acting like some kind of special priesthood. Yes, people try to take advantage of them, just like they try to take advantage of everyone else. The answer isn't to brand every contest, audition, or consult project as immoral and exploitative.