No, its quite obvious that you need to opt out if you don't want to pay -- they explicitly say the membership will be billed to the credit card attached to your account at the end of the free period, and force you to attach a credit card to your account if you don't have one already attached in order to sign up for the free Prime trial -- when you sign up, though its also easy to forget afterwards. But they give you a complete refund anytime in the paid period if you ask, as long as you haven't used any Prime-exclusive benefits during the paid period.
And its a zero-friction, click-a-button, cancel-and-get-refund thing.
I've also never seen a default behavior of enabling prime.
I don't know, maybe I read too much into it - but when I signed up, I was aware that I was both getting 30 days free, and that I would pay for it after. And I don't recall that I had to read any fine print to glean that understanding.
Actually, the free trial period with automatic billing at full price at the end of the period is a fairly common model that people in the US are used to (even if they don't like it); either it or the very similar discounted introductory period with automatic billing at full price at the end of the introductory period model is used for almost every periodic membership-based service that exists. So, I doubt very much that your statement about what most people are used to is accurate. (It may be what your experience genuinely was and the basis for your expectation despite the very clear explicit statement to the contrary that Amazon provides with free Prime trials, but I don't see any reason to believe, and have plenty of reason to doubt, that it is generally the case.)
And Amazon makes it unusually easy to cancel it for a full refund after the trial has expired and the full membership has been charged.