The taxi analogy doesn't take into account which party can make a claim against losses incurred by the employer of the victim.
It could be that City had Sala registered and his contract was valid (in which case they may owe Ł15M to Nantes) and City then have to take action to recover the Ł15M from an insurance policy taken out by the club or take action to recover monies from the party/parties held responsible for Sala's death. Other possibilities and complications are possible and it sometimes happens that several parties share culpability.