"Pay as a gift" strikes me as a self-regulating thing, exactly in that people need to understand that they forfeit PayPal protections when they do that.

Ideally, the rules shouldn't refer to specific external services, because those are a moving target. PayPal can change its rules, or some other system can become more popular, or some subset of APUG collectors may work with Westlicht Auctions, or who knows what, and we don't want to have to do someone else's police work or to adjust our rules every time someone else's rules change.