If you accept a billing agreement with a merchant on your PayPal account, that merchant becomes able to charge your PayPal account without confirmation.
Some merchants encourage or force a billing agreement before the customer can make a purchase. The PayPal UI does not make a strong distinction between entering into a billing agreement and making a standard purchase. For users who are not familiar with the PayPal checkout process, the billing agreement UI looks just like a normal step in the process.
I actually happened to be going through the Paypal settings yesterday as I couldn't get it to make a purchase on ebay and thought something might be wrong there.
I had around a dozen merchants who were listed in the automatic billing payment list and only one of them was a subscription I remember setting up. (the others were all legit and large businesses and none of them have charged me, but they could have!). I have since 'deactivated' all of them.
I really do hate Paypal but I often choose them when I buy something from a smaller web shop as I do not trust the web shop to keep my card details safe...
