I think it's because of a deep cultural divide. EU users who are big fans of the GDPR genuinely admire the law, both its intent and implementation, and also have a very positive view of the government. They believe that regulators will try to help companies comply, and will only fine as a last resort. Whereas in the US, we tend to be fairly skeptical of government. And as a consequence, since EU users think this law is a good thing for user privacy and the world at large, they want to see it applied globally by any means necessary.

But aside from that, I can't understand how EU users are unable or unwilling to separate the intent of this specific law from the broader principle that it represents, and how other countries might misuse this principle.

If any jurisdiction in the world can pass a law no matter how ridiculous that forces any business in the world with a website to comply with, on the chance that a user from that jurisdiction might stumble on that site, AND there's any kind of enforcement mechanism, then the Internet will cease to exist. Either that or become ultra-balkanized, where every user has an identifier that will ONLY give them access to sites which are fully compliant with their jurisdictions.

What if the US to passes a law that Americans are too fat and are no longer allowed to be sold gelato (they're allowed to buy gelato, but no longer allowed to be sold gelato), and then levy a multi-million dollar fine against every gelato shop in Italy where Americans visit on vacation?

That makes as much sense to me as this does.