No, what pisses me off about this is that it's plagiarism. The unsigned Jamba Juice ads look like they're Rees's work. Even a sophisticated person who's familiar with both Rees and Jamba might mistake one for the other.
This may or may not be illegal -- you could argue that Rees has a common-law trademark claim against Jamba -- but it is certainly unethical. It's sleazy. It fools the public into thinking that Jamba Juice has an endorsement that it just doesn't have.
It's easy to get caught up in property talk here and declare that Rees owns clip art, or clip art with funny speech bubbles, or what-have-you, but Rees isn't doing that and we shouldn't either. We don't need to invent exotic new copyright laws that confer ownership over clipart to condemn Jamba Juice: we can make recourse to the tried-and-true principle of not tricking people into buying your products and not plagiarizing other people.