Tim Kreider on being asked to work for free:

People who would consider it a bizarre breach of conduct to expect anyone to give them a haircut or a can of soda at no cost will ask you, with a straight face and a clear conscience, whether you wouldn’t be willing to write an essay or draw an illustration for them for nothing. They often start by telling you how much they admire your work, although not enough, evidently, to pay one cent for it. “Unfortunately we don’t have the budget to offer compensation to our contributors…” is how the pertinent line usually starts. But just as often, they simply omit any mention of payment.
Reminds me of Harlan Ellison’s strong opinions on this topic.