...well yeah...
If a US based company (via their websites) collects data on citizens in the EU, they have to comply. Otherwise the EU can issue fines. This is why some websites are geo-blocked.
- https://reutersinstitute.politics.ox.ac.uk/news/many-eu-visitors-shut-out-us-sites-response-gdpr-never-came-back
- https://www.eqs.com/compliance-blog/biggest-gdpr-fines/
If you are a website admin and know some of your traffic will come from the EU, you have to comply with the GDPR set for their residents, or block anyone from that region from accessing. You have complied by taking one of those actions.
Pretty sure you can see their email address. This should give you the opportunity to message them stating you'll be canceling the subscription. They'll still be able to subscribe on their own.