People walking towards each other on the sidewalk usually subconsciously move out of each other's ways. But there's a hierarchy to these interactions that you're probably only aware of if you're at the bottom of it. White people tend to resist deferring to people of color. White women will rarely defer to anyone, expecting everyone to get it of their way. People of color will defer to white people, etc, etc.

If you break this subconscious hierarchy, people notice and assume you're being rude or weird. Like if you move enough out of a white women's way so that she, too, can move a little out of your way so that you both avoid each other (like equals would do), she might just walk into you. Or cuss you out for being rude. Or when I, a white man, defer to a person of color, it trips them up for a second.

Since this hypothetical grocery store is nothing but "don't break unspoken rules about rudeness or you get kicked out" it means that a black person would need to act meek and submissive in order to avoid scrutiny and thus be able to stay. Meaning the rules would be more stringent against people of color, thus less people of color would be accepted, thus justifying their usual exclusion.

Ableism is super easy. Since this hypothetical prioritizes convenience over people, if you're slow at something or need more accommodation, you'd get kicked out.

Because the person in question might be an immigrant from a country where our supermarkets aren't normal, or have a neurological disease you can't see.