Speaking from a legal standpoint, mainly because it happened inside their doors.
I wouldn't necessarily agree, from a moral point of view, but I won't be surprised if that's the jury's determination and they decide to teach Wal-Mart a lesson. Fat chance of getting that far, though -- a jury would probably award $10 gazillion.
The lawyers should go looking for the numbers of Black Friday incidents at Wal-Mart where crowds got out of control. I'd bet there are several where people got knocked down.
After that, they can hardly say a trampling death was unforseeable.