Simple example, when somebody cuts into a line in a super market, how many people actually speak up? Most of the time I see eye-rolls, angry shrugs but people just move on.
Is checking out one person later going to be better or worse than what will probably be the very mild confrontation of prompting someone to move back in line? Usually, one person's inconvenient action has no meaningful impact on me at all and is quickly forgotten, but I don't know how it might impact the other person, so I wouldn't normally choose that confrontation even though it also wouldn't bother me if it happened. I'm weighing "no impact on either of us" with "no impact on me, but possible negative impact on the other."