It's like this:
We use what's called Address Verification to ensure that credit cards are not stolen. By and large, this option is only available for US card holders. Some international banks use this, the majority don't.
That extra $5 goes to paying someone to go in, disable the AVS system, manually run the charge, manually setting up your membership, making sure everything is right, then re-enabling the AVS system again. It's a pain in the ass. The platinum and knifemaker memberships aren't charged more for international customers primarily because very few international customers get those level memberships. It's a service charge, nothing more... a service charge that is waived if you can use the automated system. If you can't, then you get the service charge.
Regardless, you are talking about a difference of 6.8 cents (US) a day, versus 8.2 cents. If you don't like not getting a discount, get the higher level of membership and avoid the service charge.