Originally Posted by
seawolf
As mentioned by Whodunit68, it does not matter if UA is operating carrier, LH simply does not have the technical infrastructure in place with DHS to issue a Pre✓ boarding pass.
To issue a Pre✓ BP, the airline generate private/public key to sign their boarding pass. The private key is kept secure by the airline. The public key is provided to DHS and then distributed to boarding pass scanners at TSA checkpoints
I'm fairly sure it is only web and mobile boarding passes (i.e. anything you can generate yourself) that are digitally signed (or have to be digitally signed). Boarding passes printed by the airline do not have that requirement. I know this was why when Air Canada launched Precheck that it was immediately available at the airport but took a while for it to be available for web and mobile. WestJet is in the same situation now.
And from my experience, with an itinerary with two participating carriers, only boarding passes for that carrier's own flights will show Precheck (i.e. UA connecting to AC with UA BPs, Precheck would only show for the UA flight. You would have to see AC to get an AC BP with Precheck).
So even if LH participated, the boarding passes issued for the connection on UA would unlikely have Precheck anyways.