Just as a rule of thumb, people should not expect illustrators to work for free. Yes, there are certain circumstances where illustrators might work for free (unpaid internship/work experience, uni, charity work, work for friends) and it's one thing if they choose to (presumably, they're getting something out of that deal that they're pleased with - maybe it is 'exposure', maybe it's just kicks), but no one should ever expect illustrators to work for free.
Why? Because the arts are worth something. They have a value. And that value should be respected.
