Amazon does not hide their Prime program from members. I'm a Prime subscriber and began with the free 90 day trial. I liked the service and chose to remain a subscriber. If you miss the option not to renew, you can always go to the "MY Account" link and then click on "Manage Prime Membership". I'll attach a screen cap that clearly shows a button you can click to cancel as well as another button that says "Do not renew".
Even if you are charged the $79 fee Amazon will give you a full refund so long as you don't use the Prime service after you have been charged. Amazon is very upfront about that fee and it is certainly not an unknown service as the customer has to actively sign up for it.
Here's the link to the Prime sign up page if anybody would like to check the terms and conditions:
http://www.amazon.com/gp/subs/primeclub/signup/main.html