This is it. There is a kind of understood, cultural part that some of the other commenters are missing.

There are situations where (traditionally) tipping is expected, and that is at a sit down style restaurant or at a bar. If the restaurant requires you to fill your own drink, bus your own table (clear the dishes), or carry your own food typically Americans do not tip (this would apply to most fast food places, or places as you've described where you walk up to a counter). Do most of these places still put out a tip jar? Yes. Do most customers tip? Probably not (check the jar, it might have some token coins or a few dollar bills in it, but it will not be full).

Are you an asshole for not tipping? That depends on what the situation is. Did you just sit down for a 2 hour meal with 10 people and leave $5? Yes you are an asshole. Did you drive through Starbucks or a burger place and not put a couple bucks in jar? You are probably not an asshole.