Yvain:

The offender, for eir part, should stop offending as soon as ey realizes that the amount of pain eir actions cause is greater than the amount of annoyance it would take to avoid the offending action, even if ey can't understand why it would cause any pain at all.

In a world where people make decisions according to this principle, one has the incentive to self-modify into a utility monster who feels enormous suffering at any actions of other people one dislikes for whatever reason. And indeed, we can see this happening to some extent: when people take unreasonable offense and create drama to gain concessions, their feelings are usually quite sincere.

You say, "pretending to be offended for personal gain is... less common in reality than it is in people's imaginations." That is indeed true, but only because people have the ability to whip themselves into a very sincere feeling of offense given the incentive to do so. Although sincere, these feelings will usually subside if they realize that nothing's to be gained.