Bill Watterson on licensing...

"The world of a comic strip ought to be a special place with its own logic and life. I don't want some animation studio giving Hobbes an actor's voice, and I don't want some greeting card company using Calvin to wish people a happy anniversary, and I don't want the issue of Hobbes' reality settled by a doll manufacturer. When everything fun and magical is turned into something for sale, the strip's world is diminished. Calvin and Hobbes was designed to be a comic strip and that's all I want it to be. It's the one place where everything works the way I intend it to...

My strip is about private realities, the magic of imagination, and the specialness of certain friendships. Who would believe in the innocence of a little kid and his tiger if they cashed in on their popularity to sell overpriced knickknacks that nobody needs? Who would trust the honesty of the strip's observations when the characters are hired out as advertising hucksters? If I were to undermine my own characters like this, I would have taken the rare privilege of being paid to express my own ideas and given it up to be an ordinary salesman and a hired illustrator. I would have sold out my own creation. I have no use for that kind of cartooning."

p. 10-11
from The Calvin and Hobbes Tenth Anniversary Book

And here's an interesting fan website detailing the history of legally produced Calvin & Hobbes merchandise