As a web analyst, I can tell you that in general old, deprecated content does not get very many visits except from spiders. I would not at all be surprised if the marginal ad revenue is break-even compared to the extra server load.
Note also that they're not just throwing the traffic away. It's a decent 404. It's not the best I've seen, there's room for improvements, but it's directing people straight to the archive so they can look for the story they were linked to.