To start with, I want to emphasize that my main point was not that doing creative work for free or for exposure is wrong. As I said yesterday, I've done some creative work for free myself, and I even have a friend who is doing pro bono creative work for me (although I did offer payment for it, and would be willing to pay, if she wanted).
My main point can best be illustrated by the following story.
In his autobiography, Isaac Asimov told of the time a woman of his acquaintance asked him if he would take on some volunteer project for the community. From what I remember, Asimov said he would have been fine doing the project, but then the acquaintance went on to say that she would have asked Dr. So-and-so, but Dr. So-and-so was a very busy man.
And that statement stopped Isaac Asimov cold. He was incensed that just because he was a writer, this acquaintance assumed that he wasn't busy and had plenty of time. What bothered him was her unwarranted assumption about his life as a freelance writer.
And that's what bothers me about Google's approach to the artists mentioned in the article. It's the assumption that of course an artist would be happy with exposure as payment. Because it's not Google who gets to make that decision – it's the artists.
In the end, I'd go back to Google and anyone else offering naught but exposure, and ask them this – who exactly do they think is going to pick up the slack and pay artists if word gets around that they'll work for exposure? At what point would Google consider an artist's work to be of value? (As sethg_prime noted in the comments, Google surely paid Scott McCloud to create the online 39-page comic book introducing Chrome to the world. If McCloud's art is worth paying for, why isn't everyone else's?)
Expectations need to be set accordingly.
(As an aside, there's a fascinating chapter in the book Predictably Irrational by Dan Ariely about social norms, and how we keep them separate from market norms. Ariely has placed an excerpt from that chapter here, and I encourage everyone to take a look.)