It's great to remember that everyone only sees their preconceptions of people, but it's impractical not to judge people at all. Without judgments, there's no basis for interaction. Without judgments, people wouldn't be able to have intuition on whether to trust a business transaction. There's no way to have enough concrete proof in every transaction to conduct business without some level of trust. How does one know whether someone is trustworthy? It's a judgment call. There's a fine balance between being rigid in preconceptions of people and not judging people at all, even with being mindful that it's all happening. Since all of this is happening in real time, there's bound to be fallibility in the system. Knowing that no one can see every complexity of a person doesn't change much of human interaction because keeping that in mind to the maximum level possible would make human interaction very difficult to conduct.
Mindfulness isn't really about stopping behaviours, it's about recognizing them and seeing them for what they are. For example if I'm feeling impatient in traffic is it making the cars move faster or is it just putting me in a bad mood ? Mindfulness is about recognizing that there is no use in feeling impatient and just letting it pass. Same goes for getting worried, sometimes it will help us and sometimes it will harm us. Sometimes we get worried about our health and going to the ER was the right thing. Other times someone with anxiety may go every week because they convince themselves something is wrong. What this particular lesson wants people to see is that there is a person, even though we might not know the details, with a whole history about them. They likely have desires, dreams, triumphs and traumas. They are more than just the cashier some people ignore while talking on their phone. More than the Barista trying to greet a customer who ignores it and just says their order in hopes of getting to the next scenarios faster. If we ask ourselves what all these people really want, it seems hard when we first think we'll they're all so different ! Except when it comes down to it most of these people just want to be acknowledged in the moment, and treated well. Which is what we do if we are being mindful, we live in the moment. Many couples get to a point where they are on autopilot and stop being in the moment with their SO. Many families can do that as well, although they might share time together, it's possible they won't talk about their lives or connect. Then one day they notice they barely know the people in their lives. At some point maybe they thought they knew everything so they stopped being mindful and then one day the person was totally different. So it's not that judgement doesn't have a time or place, it's that we overuse it. Being mindful just tells us in some scenarios that the cashier or Barista who we previously didn't think was worth our time actually is at least worth a nice greeting.