It's not even plausible deniability, it's literally "how are we supposed to know what location people are in other than ISP?" That's the de facto determinate of location online. The government hasn't taken the position that a bar checking driver's licenses to confirm someone's age provides "plausible deniability", but rather that they're doing their part to comply with the law, even if some users are going to use fake IDs to cheat the system.