So, yes, you can customize Windows installs, but it's much more expensive to do so in any legal way, since you need an enterprise license, which really does cost ridiculous amounts of money. There is no cheap way to get customizable Windows. Even then, it's a bit of a hassle compared to Linux.
Microsoft and Apple are poor choices unless your (sysadmin, IT, and staff) time isn't worth anything.
Those are just the direct costs of compliance. The indirect costs of Microsoft's licensing model are something that even fewer users realize. You can't customize a distro and legally release the result to anyone outside of the organizational unit holding the license. You can't slipstream updates and legally distribute to outside parties. You can't create USB bootable media and legally release it to anyone else. Rescue discs and installation discs customized for particular hardware are left to the mercy of your OEM. All of these restrictions cause considerable friction which slows down the agility of your business. If nothing else, it makes it very hard to outsource IT functions; at most, you can hire contractors who have to keep your OS software bits separate from everyone else's OS software bits. How can this situation possibly compare favorably to free software where anyone can create and share anything? It really can't.
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.