I agree that you can't pass judgement based on so little.
I have a close family member who has a form of autism. Sometimes he can have interact with others, but sometimes it's like shutters come down and he is literally incapable of communicating and seems genuinely unaware anyone else is there (like, when he finishes eating, he'll stand up and try to walk out of the restaurant and go home, even if I'm still eating). I am a very social person and love to chat constantly, and I find spending time with him when he is in "locked in" mode quite upsetting. If the two of us are alone and he goes into locked-in mode, I will put in my iPod and go on my phone, without any guilt -- I'm certainly not going to spend an entire meal staring at the wall in silence with someone who acts like I'm not there.
Obviously this is an extreme and probably very unusual example, the point is you don't know what circumstances go on in anyone else's life.
Also, I agree that we do make all kinds of silent (sometimes subconscious) judgements about people we see; we can't stop ourselves from having thoughts. But I think there is a difference between a judgmental thought entering your head, and starting a public thread online about it to denounce them. I mean, whenever I see someone wearing a certain item of clothing my brain goes, "eww" but I'd never start a thread saying, "People who wear X are tacky and awful."