P.S. I deleted my post after a minute or so because I thought I was being irrational, making such a sweeping generalization at such an innocuous stimulus. Though that's still how I felt.
That sort of tight-lipped expression can be from a couple different emotive states (at least in my experience). It can be a very faint smile/:| face indicative of commiseration with or acknowledgement of another person in a situation that's not very positive. It can also be more of a teeth-clenching slight grimace where you're acknowledging that an uncomfortable situation exists but don't have a solution to it.
Passing by a homeless person begging for money is an excellent example of the latter in action: you consciously acknowledge that that's a real human being in a bad situation but also that there's nothing you can realistically do to make a meaningful change in their life. It's not indifference. If the person were indifferent to the suffering of others or didn't conceive of homeless people as being real people, they wouldn't feel uncomfortable not looking at them or talking to them. It's a very American thing, really--the discomfort is a specific result of not being able to meaningfully engage in the sort of silent acknowledgement or small talk that would normally pass between strangers.
You might also see that sort of expression if you're at a family reunion and racist old Uncle Larry starts going off on a tirade after having a few too many beers. Or if you see someone beating their kid, or cops harassing someone for no real reason, or someone ranting about some political bullshit, or-
You get the point. The first type is more in line with, say, greeting someone you know in a hospital after a mutual friend or relative was in a car wreck. Or while commiserating with someone you don't know very well who has had a bad day.