I think it is much like a choice of attitude. I think it is generally agreed that overall women are not as risk-taking as men. Negotiating pay involves risk - it involves the risk of being turned down, og giving a bad impression of yourself. Of course it also involves benefits, namely a higher pay. This is of course especially true if done wrong. At any rate its an extra effort compared to just blindly accepting the deal you get.
For this reason, I don't think there's any wrong in people negotiating potentially getting a higher pay - be it men or women. It's a personal choice whether you want to run that risk or not. Given the nature of markets, there's nothing strange about individuals being able to earn more by taking on more risk or doing an extra effort. In fact it seems unavoidable.
Given my experience with HR departments etc. I think it is pretty obvious that the "offer" Reddit is going to give is highly unlikely to match the individuals skills. IT is an area where higher skills can give extreme multiples in added productivity, even though this is never shown at the pay slip. So it is notoriously difficult to value someone based on just average tables based on years of eduction, experience etc. Often the true value of an employee can only be assessed some time after hiring. This is where negotiation comes in (and here I'm also talking about post-hiring salary adjustments) , because it allows the higher skilled workers to try and get some compensation for the greatly added productivity they bring, beyond what the "averages" indicate. If Reddit eliminates this aspect, they will either have to give up hiring from this pool (because their initial offers are only around average and everyone knows negotiation isn't possible) or they will have to give very high initial offers, meaning they will overpay a lot of people.
Of the two alternatives, the latter is the safest since you can always get rid of those who you turned out to be overpaying, whereas if the initial offer is too low, you are never going to see the top talent at all.
But besides from that - isn't Reddit just a trivial news web site? It's hardly a technological marvel, any semi-skilled person could quickly code up something like that. I suspect it was done only by a few people initially. Then a few truly skilled people are required to scale it up to handle the high load, but that's it. 5-6 good developers ought to be enough. I don't know how many employees they have, wouldn't be surprised it's in thousands given they have poor enough owners/mangement to higher Ellen Pao as CEO.