Recital 23 [1] of the GDPR excludes most US-based businesses from compliance with GDPR. It essentially says that sites that don’t “envisage” (their word) offering services in the EU are in fact not offering services there for the purposes of the GDPR and are thus are not subject to it. It also explicitly states that the mere accessibility of a foreign-based website from within the EU does not by itself subject the site to GDPR.

So while blocking the EU isn’t required, the other tests they use to determine whether or not you intended to offer services to EU residents are a bit murky. In light of that, what better way is there to make your intention to not serve EU users clear to all than to block EU users? That’s the main reason to do it. This kind of blockade will not prevent all EU users from accessing your site, but it doesn’t matter. You’ll have made your intent to not serve EU users clear, which will preserve your immunity to GDPR.

[1] http://www.privacy-regulation.eu/en/recital-23-GDPR.htm