It's the “deep pockets” effect, a common abuse of the court system that usually isn't seen on this scale.
A more realistic example would be someone in a phone booth that is hit by a drunk driver, and suffers serious injury. The drunk driver is the one who is responsible for the injury, but here's little likelihood of ever being able to successfully sue the driver and collect nearly enough to pay the resulting medical costs and other compensation that the victim is rightfully due. So instead, he sues the phone company that owns the phone booth. His lawyer might argue that the phone company was negligent in putting the booth so close to the street, where it might be in the path of a drunk driver, and that because the phone company is therefore at least partly responsible for the mishap, and the phone company can effort to pay what the victim is due, that the phone company ends up bearing that liability.
I have a distant memory of a ballot initiative here in California, that passed overwhelmingly, and was supposed to put a stop to this, but I am of the impression that it has failed to do so.
Anyway, from that point of view, this case seems pretty clear. Mr. Silverstein owned this very expensive complex of buildings that was destroyed in the 9/11 attacks. The suit claims that his losses, as a result of that attack, amount to more than seven billion dollars; and if so, he is rightfully entitled to be compensated in that amount. But who is going to pay for it? The actual terrorists who hijacked the planes and carried out this attack are all dead; and even if they were alive, they certainly wouldn't have anything close to several billion dollars among them. Al Qaida is in disarray, probably well outside of any jurisdiction that is vulnerable to prosecution by our nation's courts, and they probably don't have anywhere near that much in assets either. So he's going for the “deep pockets” that are within reach. His lawyers will surely argue that the airlines were in some way negligent in allowing their planes to be hijacked, and that this makes them at least partially responsible for Mr. Silverstein's losses that occurred when those planes hit his buildings.
The claim isn't nearly as outrageous as the OP makes it out to be, though if I were on the jury, it would not be easy to convince me that the airlines should be held at all liable; or to move me from the view that the airlines are victims of the attack as much as Mr. Silverstein is.
It seems somewhat worth noting, I think, that if these planes had crashed into Mr. Silverstein's buildings due to pilot error, mechanical malfunction, or other issues not related to a third party causing the planes to crash; that there would be no question that the airlines were liable for Mr. Silverstein's losses.
That Mr. Silverstein is not the only victim, or the most tragic victim, doesn't diminish his losses or the compensation to which he should be entitled.
It also now occurs to me tho think that I wouldn't be surprised to learn that Mr. Silverstein himself hasn't been the target of many similar “deep pocket” lawsuits from the families of people who were killed and injured in his buildings when the 9/11 attacks occurred. In fact, I think I would be surprised if he has not been so targeted.