I think the main problem with it is because artists are just trying so desperately to get a job in what they're passionate about they sort of blur the lines between work and leisure. To get asked by a big company to do a picture is a big thing for some people, especially when those people are just like "hey this could be my lucky break, I love doing it anyway". Companies prey on this sort of thought process and just take advantage. But to be quite fair, don't allow yourself to be taken advantage of.
Personally, I never do anything for anyone unless I'm getting paid for it. Of course, if a friend asks a favour I will do whatever they need; but outside of friends and family, fucking good luck getting me to do anything.