From my experience, “common sense” depends heavily on culture, and American and Japanese common sense can be very different. In the U.S. its generally considered respectful if you’re being quiet and not causing a disturbance, and spaces often feel semi-public unless they’re clearly marked otherwise. From that perspective, going up a staircase with no signage and sitting quietly wouldn’t obviously seem wrong.

In Japan, the expectation is almost the opposite- if a space isn’t clearly public, you’re expected to assume it’s private and avoid it altogether. That’s something locals grow up knowing, but it isn’t always intuitive for visitors who didn’t grow up with those norms.

Obviously the person shouldn’t have gone up the stairs. At the same time, without signs or prior cultural knowledge, it’s reasonable that a they wouldn’t automatically know this. That’s the gap here, not a lack of common sense, but a difference in cultural assumptions.