"international presence" can be as simple as a localized site for your German users. Then you are under GDPR etc. Sure you can ignore it if you are irrelevant but that doesn't change the fact that this is how laws work.

You’re under the GDPR if you’re subject to the GDPR’s laws describing who is subject to the GDPR.

Does American law apply globally if it says it does, absent any treaty making it so?

It depends on what exactly you mean by "apply".

For practical purposes, what really matters is whether it can be enforced. So, for example, will your country extradite you if Americans demand it? Or, say, if you travel, will any of the countries that you pass through extradite you?

The reason why DPRK is a particularly bad example is because neither is a concern just about anywhere in the world. But for large and powerful political entities such as US and EU, it is a very real concern.