I get the free trial most every year, only time I use Amazon. Last time I got it I had this issue, it turns out you have to confirm your cancellation, at the bottom of the page where it says you are unsubscribed there is a little link saying "confirm my cancellation" or something like that and there is some fine print on the page explaining it, they make it look like it is survey or something.
If someone is getting unknowingly charged they probably are not good with money in general and never look at their bank/credit card statements/accounts.
Amazon is king of dark patterns so there may be some more aggressive trick they've used to get you to technically "agree" to the renewal? It seems an obvious endgame for prime that they'd make it impossible to cancel. It's lost all its value, so the only way for whoever's in charge to keep their numbers up is to use progressively more trickery. It's funny because 6 or 7 years ago I voluntarily paid for it and considered it a great value.