I have to say it makes no sense to me in this particular case. The employer sending him on a business trip arguably had nothing, in terms of responsibility, to do with his decision to engage in what was risky behaviour on his own behalf outside working hours. What, for example, if the guy had died during a dodgy asphyxiation sex game?
French law says that, as his presence in that city was due to his employer requiring him to go there, they are liable. It doesn't make any distinction between different misadventures that could befall him, and his employer knew that. Essentially it's just a risk imposed on their business - they are effectively being required to insure their employees' lives while those employees are diverted from their normal lives by the requirements of their work.
I am not convinced that it's the best approach to take, but nor does it seem unreasonable or unjust - it's part of the law, and the employer should be aware that it is a risk they are liable for.
It's interesting to me that people seem to think that death during sex, or due to sex games, is somehow fundamentally different from other death by misadventure. It isn't, and I don't see why it should be. If he had decided to take a hot air balloon trip and fallen from the basket, the employer would also have been liable - and the case would have been no different in any substantive sense - but it probably wouldn't have made the news outside France. Sex sells newspapers.
The principle that the employer is liable for misadventure of all kinds while employees are on business trips is arbitrary, and can lead to some odd outcomes. But it is no more arbitrary than any alternative legal principle; And it is perfectly reasonable, as long as all parties are aware of the liabilities and risks that they are taking on. It assuredly does NOT imply that the employer should or could have a say in the choice of lawful activities employees might engage in while 'off the clock' on a business trip.