I can see the other side of it though - (roughly speaking) money changes hands -> definitely a contract, non-monetary licence -> probably a contract. IF they don't use CC-BY-SA images often, it might be cheaper/less risky to just offer a token payment rather than getting their legal team to check the CC licence.
Yes it makes perfect sense, not saying it doesn't. Just sharing my experience as a counterpoint to the other people in this thread who are surprised that anyone might ever pay for freely licensed content.
