I recently travelled from Paris to London on the Eurostar. After passing through the juxtaposed passport controls, passengers go through what appears to be regular airport-style security including a bag X-ray.
I was surprised to see this was staffed entirely by French customs officers, rather than private contractors (like at St Pancras), or UK customs officers (which would make sense entering the UK).
Why would the French border police care about customs checks for passengers leaving France?
Note: I’m specifically asking about customs here, not the juxtaposed passport controls