You have Apple Pay on both iPhone as well as Apple Watch - because of that, you have greater security enforced (e.g. you have to have a PIN enabled on each device). There's also the secure enclave in the iPhone.I still wonder when/if we will ever have the ability to update the Apple Watch through iTunes instead of through iOS interface.
If we see similar measures put in place with the new Mac's that support Apple Pay, we may see the capability to do this with the watch over wifi - but you'll have to have auto locking and a password enabled. No auto-login enabled. It would need to be so to ensure security.