What's happening implicitly in many of these cases, including the example above, is the establishment of a currency exchange, where the currencies being exchanged are different types of dis/comfort. This is easiest to describe with the help of an example, so I'll just keep using the one I started. But first, a brief clarification: I'm defining comfort and discomfort as inverses of each other; it depends on one's point of view, two sides of the same coin, etc.
The reason an exchange is implicitly created is because, if the passenger took a piss, the resulting discomfort of the other passengers would not be a consequence of their own bladders becoming more filled with urine. Their discomfort would, instead, be caused by the breach of social norms that the pissing passenger selfishly accomplished. In another scenario, a passenger who opted to postpone pissing until they reached a restroom, while six other passengers rode along obliviously, would essentially be saying "one bank note of my urinary tract discomfort equals six bank notes of an average person's comfort associated with things progressing in a socially acceptable manner."
We've dissected this a bit so far, but there's more to uncover. Let's look even closer...
Viewing the dis/comfort exchange in terms of an actual currency exchange becomes problematic once your observations of detail require that you peer through a microscope. One of the first discrepancies you may encounter is that the deterrent to piss when three other passengers are present equals more than half the deterrent when there are six other passengers. In fact, there's hardly a deterrent reduction at all. This is very different from the bank, where the former of two pieces of paper worth three and six bank notes, respectively, is valued at exactly half the latter. The sleigh of hand in the preceding is inconsequential; you'll obtain similar results if the bank notes retain their original metaphorical attribute...the number of other passengers has no bearing on the discomfort resulting from being polite.
As humans, we have the wondrous ability to regard ourselves from the point of view of another. This is especially easy when the other person is within our line of sight, a circumstance that encourages the notion of seeing ourselves from another's point of view to be taken quite literally. Such occasions are worth mentioning here because they introduce a relevant quirk. Though I somewhat trivialized the endeavor by describing it as 'especially easy', an aspect of it that's more involved is to ensure that when one is viewing oneself from the other's visual perspective, insider knowledge doesn't infiltrate the perception.
By this I mean, for example, if two people, Ashley and Philip, are separated by 20 meters and approaching each other on the sidewalk, and Philip is imagining how he looks through Ashley's eyes, he should take care not to allow his knowledge of what he's spitting onto the sidewalk influence his out-of-body perception of himself, since Ashley is still too far away to determine whether he's spitting phlegm, which is possibly offensive, or watermelon seeds, which could only offend someone who is sour to begin with. This demonstrates how people are liable to erroneously incriminate themselves for being impolite when viewing themselves through another's eyes.