I'm biased for the GDPR, since I think every site should follow its principles regardless of legal obligation, but I don't think the rationale you're proposing is scalable.
Its your favorite dictator, the leader of Crazystan and from 29th of May 2018 I ask that from that date, for every site accessed by citizens of my country I require the hosting company to send one employee to be sacrificed to our mighty gods.
Failure to comply will attract a fine of 50 Gazillion dollars.
I'm not even sure about sane way to map IP address to country. There are some geolocation services, but I doubt that they are 100% precise and probably paid. Also if I'm using geolocation service passing IP of the incoming request, does that mean that I'm already violated someone's privacy? This is weird.
That being said, the starting point shouldn't be, "there's no need to imagine that I'm violating GDPR. I only serve Americans". The starting point should be, "I had better imagine that I might be violating GDPR even though I only intend to serve Americans. Are there things I haven't considered? Are there resources I should seek out? As a service provider of some kind, hadn't I better spend a day or two imagining the ways I might run into trouble and plan to avoid it?"