From a full-service restaurant's point of view:
If the bar is used as another section of the dining room, and the bar stools need to be kept on deck in a certain way (i.e. side-by-each to accommodate deuces who will arrive later), and diners sit themselves down at the bar and begin their meal period without any guidance from the restaurant as to seating, then it should be up to the restaurant staff to move these diners in an appropriate fashion when the time comes to make more space. I don't think this applies in a drinking-bar situation, where it's every man (or woman) for themselves.
It should not be left up to Guest A to ask Guest B to move, because then you encounter the nastiness mentioned in the original post, and as far as I am concerned, the host has shirked their duties. The problem can be pretty much eliminated if there is communication between the restaurant and the guest during the initial seating. Menus should be placed in front of the bar stools and guests should be clearly directed to the bar stools so that they will not have to stand up and move later. A host who seats their bar haphazardly (and in the same vein, a host who seats deuces at four-tops) deserves the clusterf**k they're going to get later when there are too many butts and not enough seats.
Also, if a bar is used as another section of the dining room, and guests arrive who wish only to drink (or as we call them, "just drinkins"), I don't think it is unacceptable for the restaurant staff to say something like: "please be welcome to these seats for now, but it is likely that in the future people will arrive who might wish to sit here and dine (or in the case of reservations, people who have reserved these bar stools to dine), and the worst thing that could happen is we might ask you to finish your drink on our patio, and your drink will be on us" or something along those lines. In 10 years of working in the industry I have never had an issue with a guest who didn't understand this scenario--as long as the scenario is established up front and immediately. Full disclosure on part of the restaurant. In the original post the restaurant staff was clearly at fault for not taking control of the seating aspect from the get-go. And it sounds like the other guests were assholes, but that is beside the point.
From a different angle, in reservations-based dining, if the restaurant is running behind on table seating, and guests waiting for late reservations are clogging up the bar stools, it is NOT acceptable for the restaurant to ask these guests to stand to accommodate bar diners. Ever. You suck it up, and admit to your bar diners that you are running behind, and you offer an alternative (like free bubbly for everyone, including, most probably, yourself). I don't like restaurants where Guest A is forced to seat-stalk Guest B at the bar--this is why I can not go to Big Star. I need a host with a firm hand there to control the seating action, bar included.
Back on topic, asking a guest to move down at a bar should be done at an appropriate time in the meal period (preferably between courses), and it should never be done without some kind of compensation for the guest--a glass of wine, dessert, etc. It doesn't have to be all touchy-feely, nobody has to kowtow to anybody else, but it should be recognized as grace on the part of the guest, and a grace which should certainly be returned on the part of the restaurant.
In my experience, a lot of guests notice when they might need to move and move without being asked. Bless them.
EDIT: to say that personally, I am a seat ninja and can sense needing to move before it happens. BUT. If for some reason I didn't notice, and the person who asked me to move was A). an asshole, and B). disruptive, the reaction they would get from me would be a big fat stink-eye and they can sure stand and wait until I am done.