I'm just not entirely sure how someone can be taken advantage of at a hackathon. Are you implying that someone will steal their product? Or that they'll somehow be convinced to give up a large % stake in their project by someone who can't code? Again, an example of a hackathon where this has happened would be great. Besmirching the name of hackathons in general is wrong.

As for hacker mansions, the post you linked to opens with "Like the idea of living/working in a Palo Alto hacker mansion?". If the answer to that question is no, then the likelihood is you wouldn't apply to this job.

I disagree with your assessment that this ad is "clearly written by someone who thinks they can throw material things at a Developer and then get them to work 20-hour days for three months straight". From my perspective, they're offering an amazing summer experience for the right candidate who wants to work with them. This ad is clearly not aimed for experienced developers, but rather young folk who are looking to get experience in the startup community.

Let's not forget that employment is at will, so if someone is not enjoying working at such a place, they can leave at any time. I, for one, know a bunch of people who love and thrive in such environments. As for "20-hour days", that's just a number you've thrown out. Who's to say you couldn't have a great work/life balance at a hacker mansion?