How can it be a trade barrier when EU companies are more affected by it? (They have to provide these protections for everyone, whereas non-EU companies only have to provide them for people in the EU).
You just answered your own question. The cost of compliance is largely a fixed cost. So if only 50% of my users come from the EU, then my per-user costs are 2x what an EU-centric company's would be. So it skews the economics in favor of blocking the EU if your business is not EU-centric. This in turn means users are pushed to EU companies that have no choice but to comply.
But the cost is the same for both companies inside and outside of EU, so US companies "not being forced" just mean they have the luxury to decide whether EU customers/visitors are a concern to them - EU companies HAVE to do it, even if 95% of their userbase is in the US (but I think those US users will appreciate it still).
Imagine a law that forced all companies in the EU to be polite to their customers no matter what or face fines. In that case you could also say that users would be pushed towards EU companies because they were "forced" to be polite, but I think that would be deserved and US companies could just do the same. Similarly to GDPR, if one side is forced to treat my data with respect and actively have me consent, then I would chose them, law or no law.