It's pretty heavily dark patterned for me. When you go checkout there is a big prime banner inviting you to click now that looks the the default "next screen" button, and smaller fainter text below saying "continue without enjoying prime benefits".

Something similar happens again with the shipping. I only ever buy enough to get free shipping, but it never defaults to that, it tries to trick you by defaulting to paid, and then when you scroll to change your shipping to free, it again makes "join prime" the most default looking option to pick.

I'm pretty sure in the past there was an extra nag somewhere but the above is my most recent experience. Maybe legal but certainly feels like you're dealing with a scammer.