Further, logging an IP address has also been necessary for security--to detect DoS and DDoS attacks, for instance, as both involve many repeated connections from the same IP (though you can offload that to a service now).
You seem to be confusing temporarily holding information in the database (no matter whether in-memory or on persistent media) for operational purposes (be it networking or threat detection) and permanent storage of the same information way beyond the time it ceased to be useful for the purposes above.
Also, even if you only store the information for 24 hours--how many users are on Hackernews in a given hour? How many of them click through the top links? Certainly enough that some sites have crashed under the load, which makes me think that the number is higher than 50K. Congrats, you're now storing 50K IPs, despite having an automated system to delete them in a day.
(I don't even wanna think about the headache it'd cause if you got dragged to court over this. I don't know how many judges will understand that you delete the list every 24 hours to avoid being liable for storing it, not to get rid of evidence.)