Imagine someone who's never been exposed to company A or their products, but runs across company B products, which are a derivative work of company A's products.
Now imagine that this person enjoyed company B's products so much that they had to have everything that relates to it, so they go out and but all related works produced by company A. Now remember, this person may have otherwise never been exposed to company A's products if it weren't for their exposure to derivative works produced by company B.
Don't tell me this would be a rare occurrence, for this has happend to me many times with respect to music, video, and written works. I have, through consumption of products that infringe on copyright, spent money to acquire the works that inspired them to begin with.