You wouldn't be responsible for the payment just like you aren't responsible for the payment with a stolen credit card. I also wouldn't be surprised if these terminals had a pulse detector that would stop a severed hand from working at all.

You'd probably even have an easier time proving your hand was stolen it since your hand is missing.

It's still a problem that if someone compromises your biometric data they'll have full access forever. In practice that isn't a huge problem because it's still really hard to do.