The link is there. It's the Lufthansa-owned and -operated flight school in Arizona, USA that produced Andreas Lubitz. There it seems pretty clear to me that Lufthansa is exposed to possible action under American tort law.
Consider the plight of VW in the USA now.
Tellingly, in the States the opening ploy was "Here, have a thousand bucks walking-around money, just for starters, while we figure out what to do next." Here in Germany there was no question of handing out that sort of money to VW customers; many are simply stuck waiting to find out how VW should fix their cars.
Now the real legal troubles, the big ones, start for VW in the States, when it's not going to be "Oh, but that was Volkswagen Group of America, not VW AG of Wolfsburg, Germany!" The problem, as it also is for Lufthansa, is not a link to US soil, but a link between a US entity, VW Group of America in one case, the Lufthansa Arizona flight school in the other, and their respective parent companies in Germany, the big companies with the deep pockets.