This has always been the case for every itinerary I have ever searched for TATL flights from LAX for any *A airline. For whatever reason, UA does not like to include their own PE seats when codesharing to Europe. Sometimes, I am able to force it on the UA website by booking each segment individually, but that usually results in a significant price increase, or more likely, no availability for the UA segment. The only time I can get the domestic on UA in PE is if the connecting TATL flight is also UA, but even that is no guarantee. Fortunately, it is fairly easy to avoid UA entirely on TATL from LAX, so that is the simple solution for me. Unfortunately, all the LH-group airlines charge for PE seat assignments.