To carry on this discussion toward a logical end, it is not permissible to thread any connector into a pvc female adaptor unless the connector is listed for that use.
How about all of us electricians who routinely install threaded emt connectors into threaded malleable iron and aluminum or zinc condulets (like tee's, LB's etc.)? Threaded condulets are only approved for conduit with tapered threads like rigid or IMC. Only set screw type condulets are approved for emt.
How many pvc stubs under panelboards convert to emt with female adaptors and emt connectors and are overlooked by inspectors because it is not worth the fight.
My point is that if inspectors made us remove all unapproved installations in this country we would have very few working electrical systems.
If one starts looking, there are many unapproved uses of materials in the US. However, we still have the safest electrical systems in the world.
