What you've described sounds like a caricature. While it is possible that the more nuanced culture formed elsewhere and was made extreme on it's way to your shores (and, indeed, there is evidence of this having happened in many other cases), I am not sure that is what is going on here. The specific attitude you describe is one I see commonly attributed to hipsters, and only in very rare cases a fair characterization. I think that The Yeah Yeah Yeahs and Radiohead (to throw something out there) are examples of incredibly commercial, mainstream bands which most of the people you are describing would be into.
Saying that you don't like the new Shins album because it is very mainstream doesn't actually mean that you don't like it because people are buying it (they aren't), it means that it doesn't seem challenging or interesting to you. It seems designed for mainstream appeal.
In the US, the word is not applied solely to the group that you describe, either. It's used to describe people who live in particular areas, people who ride bicycles, people who shop at thrift stores, people who listen to public radio, people who see lots of live music, and people who listen to records on vinyl, and this is by no means an exhaustive list. The most important thing is that people seem to have flipped your description of the attitude on it's head; It's not about whether or not you dislike "mainstream" or "commercial" things, it's about whether or not you like something which doesn't fall into one of these categories.
I agree that your description of this group doesn't make me want to hang out with anyone in it, but it certainly doesn't make me feel like I shouldn't tollerate it. If there is a group of people who want to be very bad at conversation at parties, more power to them.
If I believed that most people made these judgements on a case by case basis, that would be one thing, but that's not the case in my experience. The assumption seems to be that if you match one of these very superficial and general categories, I can mantle your personality and judge you (with little/no other justification, see what I did there?)