Adulteration of olive oil typically happens in transit/shipping by wholesalers, not by producers. Spanish or Italian olive farms aren't diluting with corn oil or whatever during production on the farm/when pressing, it's by aggregators who are buying and selling oil in bulk by volume. So if you're buying olive made, processed, and bottled by a single farm whose origin is local you're structurally at less risk than a generic "Italian" oil or one that says the country of origin could one of 3-4 countries. Also California has a mandatory sampling and testing program https://www.oliveoilcommission.org/

If you want to go deep on the topic, this is a great read: https://www.goodreads.com/book/show/10955085-extra-virginity

Primarily because there are stricter requirements via FDA on the US food supply. If a US producer gets caught adulterating they're going to face legal issues that a foreign producer will not face. Not saying it's not possible that CA producers could be adulterating, but if they get caught there will be consequences that an importer won't face - the importer can just say they were trusting their suppliers overseas.

Also found this (from 2010): "The research team found that 69 percent of the imported oils sampled, compared with just 10 percent of the California-produced oils sampled, failed to meet internationally accepted standards for extra virgin olive oil." https://www.ucdavis.edu/news/most-imported-olive-oils-don%E2...