That's standard practice for all cards, it's not just AMEX (I believe the authority is based on the account rather than the card). In the UK, there's no easy way to cancel a recurring payment on a card other than contacting the entity taking the payments. If they refuse, you can complain to the card provider and they will eventually sort it out, but payments will still go through in the meantime.

Moral of the story: Don't let anyone have a recurring payment authorisation on your card.