For those with access to a supermarket, a combination of lack of time, lack of education, and lack of ability to delay gratification that causes people to eat junk food. Not money.
None of the above. For most poor and even lower-middle class families, the limiting factor is lack of access to food preparation equipment and facilities. Low-income housing often lacks a kitchen. Even if you have a kitchen, one often lacks appliances; trying to subsist on unprocessed food without a refrigerator or a stove is difficult to put it mildly. Families near the poverty line move from place to place a lot, often on short notice in response to evictions. There's no way they could maintain possession of bulky appliances under such circumstances, not to mention an adequate inventory of cookware.
Poor families are really living on the edge, much more than you realize. Once you get to the point where you can't afford a security deposit for an apartment, a lot of options close off. Food preparation is one of them.
Food prices are high, but all of my meals (which are nutritious) cost $1-$2 max, usually closer to $1. You just have to know how and where to shop. Of course, this is the US, which is a first world country...
It is not enough to know how and where to shop. You also, generally, need a kitchen and appliances (stove, refrigerator, etc.) in order to produce nutritions $1 meals. Many poor and even lower-middle class families simply don't have these things. The kind of housing that you can get for cheap is going to be one-room boarding houses with limited access to food preparation facilities. You're lucky to have even a shared kitchen. As for appliances, they're not actually very expensive -- an iPhone costs more -- but poor families generally move far too often (usually involuntarily) to maintain possession of bulky items.
The other two examples, however.. even if I don't personally agree with them, why shouldn't they be allowed? I think those are perfect examples of good free market. Someone should be able to sell something they make for whatever they want.
Monopoly power leads to deadweight loss and suboptimal consumer surplus. This is economics 101. The theory is very well known. I wouldn't expect members of the general public to know basic economics, but on slashdot, it's fair game.
There are other obvious examples of free market failure. Do you let factories pollute the oceans? What about overfishing and tragedy of the commons? How about photocopying books at cost -- do you prevent this (via copyright) even though it's obviously market interference?
Continuing with the Harvard theme, if you google Benjamin Pierce assistant professor, the first page of Google results links to the following former BPs: Lauren Williams, Pavel Etingof, Danny Calegari, Nathan Dunfield, and Xinwen Zhu. These people, obviously, landed on their feet and got hired in other universities, quite prestigious universities in fact. And I am sure if you did a comprehensive survey of all former BPs, you'd find the majority working in R1 universities and on the tenure-track. Similar remarks would apply to the untenured named instructorships at any other elite math department, e.g. Dickson Instructor, C.L.E. Moore Instructor, Veblen Research Instructorship, and so on. They're all slightly underpaid. They're all hugely prestigious. And few people have trouble landing a job afterwards.
If you get denied tenure at a lower-ranked school, then yes, that is a disaster. Those schools are set up to give you every opportunity to pass the tenure review. If you fail to do so, then that's on you, and as you say, you'll be an outcast.
When Mary Margaret Vojtko died last September—penniless and virtually homeless and eighty-three years old, having been referred to Adult Protective Services because the effects of living in poverty made it seem to some that she was incapable of caring for herself—it made the news because she was a professor.
The story of Mary Margaret Vojtko is more complicated than it seems on first glance. Vojtko was a hoarder who rebuffed numerous attempts by others to reach out and help. Among other things, she refused to let a repairman fix her boiler because she didn't want anyone disturbing her house. Yes, she was paid poorly and had no benefits, but there were other factors at work.
Once you get below the very top, the GP is basically right, all the way down to at least liberal arts institutions (at community colleges, the situation is again different). I'm an associate professor of mathematics at a very good but not absolute top university (Waterloo). All associate professors here have tenure. I make north of 10k gross per month, although perhaps not well north. I'm very happy where I am. I could make more money in private industry, but tenure is worth more to me than the salary difference. In more technical fields than mathematics (such as computer science or engineering), the salaries are higher, as they have to be, to compete with Google and engineering firms.
All of the above applies to tenure-track professors only. Contingent faculty positions are much more financially precarious.