Why would a VPN company care about what Utah requires if they have no legal presence there(or in the US for that matter)?

Besides, there's no such thing as "residential" IPs. If I set a VPN gate at my mum's house, how would the porn company know?

The "impossibility" is in them saying "we cannot guarantee with 100% certainty that the user isn't using a VPN" and that's correct, they can't - but the law has no provision for that, they are risking fines over something they cannot control.