GDPR has compliance costs even if you are doing nothing that violates privacy. As long as you've got data that can be associated with a user you might have GDPR obligations, including handling inquiries from users about what data you have about them, requests from users to delete their data, and similar.
Just being able to receive such requests has costs, because GDPR requires that you have a contact in the Union that users can contact for such things.
So say you've got an online forum in California and some EU people join and participate. Whether or not that is enough to make you subject to GDPR depends on Article 3(2). There's lots of subjectiveness in Article 3(2) so it is not at all clear where the boundaries are.
If it does, then whether or not you have to have an in-Union representative is covered under Article 27. You do not need one if your processing of covered data is all of the following: (1) occasional, (2) does not fall under some special categories of data, (3) is unlikely to result in a "risk to the rights and freedoms of natural persons".
That's pretty fuzzy. What is occasional processing?
If you aren't sure that GDPR does not apply, or aren't sure that if it does your processing is occasional enough to fly under Article 27's radar, you need an Article 27 representative.
Eventually there will be rulings from EU data regulators that will make the boundaries of Article 3(2) clearer, so that you may be able to stay in "does not apply" without blocking. Or maybe rulings will clarify Article 27 so you will be able to confidently determine that your processing is occasional enough to not need a rep.
There's actually a lot of unclarity in GDPR. Take the data subject's right to have you delete their data. How do you actually implement that? Writing some scripts to delete from your database is probably not too hard.
But what about backups? Do you need to go through all your backup sets and delete their data from those? What about printed records (yes, GDPR covers printed data)? Offsite long term archives?
For a site like a forum, what about data that was in messages they posted that were quoted in messages from other users?
Until all these kind of things are cleared up by EU data regulator rulings it can make a lot of sense for a site that is aimed mainly at a non-EU audience to block EU users.