Since the watch doesn't re-lock when transferring to someone else this way, anything that's accessible on the watch from the phone would be available with the victims IPhone, which will also be stolen. If you keep sensitive information in your contacts, for instance, there might be enough there to steal someone's identity. Before the watch, the phone would be locked, and easily bricked if stolen, making it a much less desirable item to steal on its own.
This really isn't about liability per se, as more of an exercise in exploiting an Apple security hole. The idea of a gang of thieves who drug unsuspecting Watch users and commit credit card fraud is one such realistic example, given that such thieves have operated this way in the past, and it's a very old con -- just look up Mickey Finn. That said, I'm not trying to instill panic, just explore a hypothetical. I certainly don't think this could likely happen to me or many others on this forum. But, to the extent it concerns Apple, it would be less about card liability for the victim, and more about Apple not wanting Watch wearers target for crime, which is the reason lawmakers have passed legislation to require phones to be "bricked".
However, to address your question here's the hypothetical I'm basing this scenario on -- it's 2016 and contact-less pay terminals are everywhere and they all accept Pay. People have slimmed down their wallets to just whatever cards and IDs that can't be accommodated by the watch, namely a driver's license, and perhaps a little cash. I know once Pay is accepted everywhere I won't be carrying a credit card anymore. At the present, I only carry one, but I have others I don't carry -- all of which are tied to Pay. I never carry cash. So let's say it's a typical mugging, the guy takes my watch, iPhone and wallet, which gives him two useless pieces of electronics, and a driver's license (though the watch can probably still be sold easily on the black market). But is that really enough to warrant assault with a deadly weapon charge if caught?
Now enter the smart criminal, who drugs his victim anonymously, robs them of an unlocked Watch, iPhone and any other valuables they may have, assuming no credit cards in the wallet. But even if there is a credit card in the wallet: we don't know how Pay will work with ATMs, but if a pin is no longer required, then there's a quick source of cash that wasn't available before. Also, some clerks might ask for an ID when using a physical card, but with Pay there's no questions asked. Also, a drugged victim can't log into iCloud and cancel Pay until they wake up. So many reasons to do it this way versus just pick-pocketing a wallet which is likely to be empty anyway in this hypothetical.