I like to think I'm pretty computer literate, yet I ended up signing up for the free trial without actually wanting to. Fortunatley I realised so I gave it a go, decided I didn't want it and cancelled it before they charged me any money. However a relative of mine who isn't so savvy didn't realise an ended up paying the £80 and is trying to get the money back (like getting blood out of a stone I think). So agree with the OP, Amazon are being extremely sneaky here.
It's more a case of what they aren't doing. They aren't asking you to confirm that you really want to sign up to the free trial. They aren't highlighting that at the end of the free trial you'll be automatically charged for a year's subscription.