I'm probably the one giving the "sideways looks" when a kid is throwing a tantrum. Usually just to gauge the situation (or ask myself: "How would I respond to this?"). I consider myself truly lucky to have a great kid, she is very well behaved (in public) and follows instructions very well! Not to say we haven't had our "moments."
Like a previous poster said: You can't blame a kid for crying when you, as the parent KNEW it was time for their nap/snack/etc!! If I have to go somewhere and I even have a thought that dd might need something, I PREPARE! (Pack a snack, drink, book, other distraction!)
I think I have only ever had to leave a place (for a tantrum/fit) maybe twice. And, I would bet that they were both because she was tired! A quick cuddle in the car, or a drive around the parking lot, and she was sound asleep. I could then decide to go home or tote her around the store while I finished my shopping.
I am also very much bothered by the parents who just ignore it--sometimes I think they actually do not hear it (maybe because it is just constant "noise" at home?). Yeah, then there are the passive parents who are all "aw, honey, stop acting up. Sweetie, you need to stop crying. Mommy asked you to please stop." Ugh!!
Sometimes it IS hard to reserve judgment, though! Just last night there were 2 kids in the store and the older brother (~5) was being rough with his sister (~2) and he threw a toy at her and then shoved her over. I heard her head hit the floor from 2 aisles over!! I jumped over, asked "are you okay? where's your mama?" and the boy is all like "uh, she just fell." She cries "my head!" and then starts wailing! The mom walks over (arms full of stuff) and gives me the dirtiest look! I told her what happened and that the girl hit her head really hard. She just stared all blank-face. I asked the girl again "are you okay" and she mumbled "uh-huh, mama." So I slowly walked away. Finally, the mom reaches down her hand to the girl "get up, what were YOU doing?!" Goodness Gracious, the words that were going through my head for that MAMA (and the brother)!!!