Winston Churchill wrote:
You need to remember that the watch is treated just like a plastic cards from the readers point of view.
This only applies for lower value payments. As Apple Watch and iPhone are not the same as plastic cards, suitably equipped terminals can treat them differently. This is what enables them (and other NFC-equipped devices with similar features) to be used for higher value contactless payments than plastic cards.
Winston Churchill wrote:
Whilst removing your watch stops the payment system working and removing wrist detection removes the card from your watch, plastic cards have no such protection. If you drop your contactless card in the street the person finding it needs no signatures, PIN numbers or the like to start spending your money, you really wouldn't want the finder to get much more than a round of drinks without more protection.
The standard limit for contactless payments in the UK is presently £20. This will soon rise (September 2015) to £30 for all contactless cards and devices.
The payment system can also support higher / "high value" mobile contactless payments.
What enables Apple Pay on Apple Watch and iPhone to use this - and what sets them apart from plastic cards - is that the devices themselves incorporate a verification method. iPhone uses Touch ID and a passcode; Apple Watch uses Wrist Detection and a passcode. This adds an extra layer of security that is similar to Chip and PIN.
Unless already supported, merchants need to update their payment terminal software in order to accept high value mobile contactless payments. This - along with the fact that the capability is still relatively new - is the main reason why not all contactless-enabled retailers can yet accept Apple Pay for sums over £20 in the UK (or over £30 after the change in the standard contactless threshold).