The most fun (well if you are low and jeuvenile like myself) thing to point out, in specific particular to someone who thinks they are so entitled to other people's volunteer work, is that they are the begger.

Begger at the free soup kitchen is outraged at the lack of truffles.

If you're company is so impressive, and you are so impressive within it, then by all means go forth and buy satisfaction somewhere. Why is Steve Jobs fighting with a homeless guy over a half-smoked cigarrette someone threw away?

In this example the guy removes all doubt about that superiority mindset by calling the developer loser.

Its a common thread with the expectations of "Well I have gotten everything for free up to this point, why don't I get all your time and energy for free too?"

I love contributing to OSS but I don't even contribute to MIT stuff just because every corporation doesn't deserve your work scot-free.

Notice the grammar and lack of capitalization in the responses, in addition to the content.

I try not to write like that even in text messages to friends, let alone in a public environment that is related to my work.

If I found this person in my organization I would have them dismissed from my company.

Do socially maladjusted people exist in similar numbers everywhere and open source is just a great opportunity to make their presence (unintentionally) known? Or is there something unique to this realm that makes them more prevalent?

There’s nothing unique. Just the thousands of adjusted people don’t say a word so the negative comments echo louder even if they are the minority.

Anecdotally, I established a website in 2005 featuring long-form nonfiction, and it gained a respectable following. After years of posting several new articles per month, I started feeling the burnout, so I took a break, and then slowed my publication rate to one new post every few months. I soon became the recipient of a deluge of nasty emails from people who were furious that I had reduced the amount of free content I provided. This small, vocal subset of readers obviously enjoyed my work, otherwise they would be indifferent to a reduction in output, yet they were emailing me imploring me to kill myself because I am a failure.

I really wonder what their day-to-day internal monologue is like.

Ask a retail worker.

Entitled people tend to show their faces on some contexts, and hide it on other ones.

What is weird is that with free software developers the power relation has the opposite direction than the one where this normally happens.

I think it's two things. We develop our intuition for how many maladjusted people there are from our physical surroundings, but we're limited in how many we can meet and are unlikely to meet the extreme outliers; on the internet everyone is our neighbor. And then on top of that, they are overrepresented because they are much more likely to be removed from communities to arrive at new ones.

One might ask the same question about the entire internet.

My favourite response, if only for it's brevity, is now on a long-lost repo. The owner replying to a particularly snarky requester:

"If I was working for a big org like you, then I couldn't tell you to go f*k yourself. But I don't, so I can."