Nothing special from a legal standpoint, but it's all about how easy they make it. I have bought from other online retailers, and whilst the experiences haven't been bad, they still aren't quite as slick compared to Amazon. I can't really point to anything in particular that will convince you, it's just that the whole process is painless and easy, not just for the basics (purchasing) but for everything.
Aside from the 'refund before the item was returned' example, I could also point to when I'd had to speak to Amazon customer support before, all done through instant messaging on the website with one of their team. Resolved within minutes. Contrast with other retailers where there's a back and forth with emails. Again, not horrible with other retailers, but the Amazon experience was more streamlined.