The mystery of why they do is a bit too common of a question for me. Because eventually, there is no where else to run besides out of the game, which is still a win and maybe the goal of the jerk.
I can think of a potential reason why-
Some people eventually get tired of running. Because no matter where they run they eventually will either run into another jerk or the same jerk again and sometimes worse than before. Especially if the jerk know that person will simply flee.
I avoided people who took an unpleasant interest (people who were pleasant didn't bug me, and sometimes gave pointers I might be able to use) in the details of my build in CoX, whenever the opportunity presented itself, and yet about the only activities that closed off for me were things I had no interest in, anyway (like speed ITFs and srs bsns farms). For anything else, the most I had to do was leave an individual team. If the person who was judging me had the star, I had no problem with them setting requirements, even if I thought they were incorrect (I'd state my case, but if they still wanted X or Y, then so be it). If they didn't have the star and persisted, I'd ask the team if they wanted me to go. Usually they would say, "no, don't listen to ____... he is a <bleep!>" and that was pretty much that. Sometimes they agreed with _____, in which case I was happy to leave.
Why was I happy to do so? From my perspective, jerks are so eager to identify themselves that it is almost like they are required by law to do so (sort like how medieval stories will feature lepers having to shout warnings to people to stay away... "Outcast! Unclean!"). Since they are nice enough to clearly warn me they are jerks, I will take advantage of that warning.
If I was to stay on a team where I wasn't wanted, I would only exacerbate a bad situation (I'd become part of the problem). Plus, why should I stick with a team that doesn't want me when there is bound to be one that does? Or, better yet, I can start a team myself so I know the star will be willing to put up with me. :3 In any case, I get to avoid hanging out with a jerk, so win.
Now, if we are talking about something more serious than having different opinions than I do about builds, IOW something I'd think they should be reported for, I do that as well. If I feel obligated to put my $0.02 in about something they are saying, I do that. But, once those obligations are done, so am I. If it is a forum, I stop replying (if all I would be doing is repeating myself); if it is ingame, I stop paying attention to them (and if that is not enough, I'll /ignore them).
I definitely don't pursue a deeper relationship with them, since they are jerks, and they have done their part by giving me notice that they are jerks. I loved when CoX added the ability to add stars and a comment to players, since it let me easily identify them in the future, as well (As a side benefit, if I had taken the time to produce a properly snarky note, I got to enjoy it again ^_^7 ).
But the million dollar question to me is Why is it expected for the target to run?
To me, that is an easy one. I realize you are talking about why other folks expect it, but since I don't think in those terms my answer will be why I expect it of myself. It is because, while I have no control over other people to make them do what I think people should do, I do have control over whether I do what I think people should do. How can I have any expectation about the behavior of others if I won't do it myself? If I don't behave like a jerk, and they do, there will be people who can tell the difference.
The side benefit of that control realization is that it comes with the realization that they have no control over me, either. If I end up bowing out of something for the reasons I gave up above, a jerk might consider that "running" but, given that they have already given me ample evidence that their opinions are not mine, why should I care what they think? They will go on being jerks, of course, but there was never anything I could do about that (and, frankly, I think they end up paying for it over and over again in their lives).
Now,
if CoT ends up having srs bsns raiding, that may be another activity that I would have to avoid. imo, the thing that inspires interest in others gear isn't so much the availability of the info as it is the belief that they are doing something hardcore enough that they are justified in being picky. In CoX that belief was often misguided (and therefore was not common, at least on the five or so servers I played), but for fancy raid games I could see how they would care.