Turns out some sellers add themselves as pre-approved billers and don't/can't remove themselves when you cancel your membership/billing agreement. You have to remove it yourself, or they could charge your paypal in the future without any advance warning. Your only clue would be a notification that the money has already been deducted from your paypal, and at that point you cannot decline/reverse the transaction by yourself.
To prevent that, remove unwanted billers from the following link:
When I checked today, I had a couple of merchants on the list who I have not used in years, and am personally aware of merchants who have fraudulently charged people I know in the last couple of days.
Paypal refuses to entertain any complaints regarding merchants if they're on the list - their argument is that you have agreed to be billed by the merchant, so any problems are now between you and the merchant. Paypal has also said that the agreement can only be ended if agreed by the user AND the merchant, which leaves the feature wide open for abuse.
The feature of being able to view pre-approved billers is new to paypal even though having pre-approved billers is an old thing. it could be that paypal added the feature in response to complaints, but their handling remains poor. Please take care of yourselves by removing billers from the list, or you might be faced with a lengthy fight to get your money back! If the balance was deducted from paypal itself you're out of luck. If it was deducted from your credit card you still have hope as you can raise a dispute there.
ps: special shout out to past users of Shaw academy (www.shawacademy.com), as they recently did this (deduct money from people who had canceled their memberships over a year ago)