Hassling Tor users shouldn't become the Internet's default. If you're having trouble, consider informing Tor users checking out that you won't process the payment without their providing additional information. This raises the cost to carders a lot more than needing to rent a SOCKS proxy in a residential area.
It's not worth the development costs and extra verification costs to try to weed a small number of legitimate purchases from a sea of illegitimate ones though.
It isn't that there is a sea of illegitimate traffic so much that their methodology is incredibly flawed, and they have little financial incentive to fix it. As a society, we have to give them that incentive, or we will lose access to a shared resource.
We can have our Internet heavily censored or heavily censored with a ray of sunshine. Is that worth the engineering costs?
