I see, thanks.
The relevant trait of open source seems to be that the authors are readily identifiable. The US OFAC decided to sanction Tornado Cash for reasons that have nothing to do with open source, and then (probably) used open source to find the most important maintainer, and sanctioned him personally.
Finding the most important maintainer of a closed-source web service shouldn't be impossible, but open source clearly makes it simpler: No subpoena required.