Going by the official forums this doesn't look as if it is going to be fixed any time soon and seeing as I've done extensive experimentation on it I thought I'd write up a detailed guide on how to use the guards to essentially lock down the bridge across the Blackstone Span and get a bunch of HKs with more or less impunity.
The way the Tol Barad guards worked changed in 4.1.
Before 4.1 they used to require someone to get within their aggro range before they attacked them.
After 4.1 if you are in combat and are within their aggro range they will attack whoever you are in combat with. This means the guards can be used to offensively attack people FAR away from their posts. They also have more powerful ranged attacks now so they don't even have to catch right up to someone to kill them. They can kill them from 30 yards off.
As Horde it is far easier to kill people with the guards because of their proximity to the bridge. I've been able to solo barricade the bridge and cause Alliance at least 3 deaths to guards if they want to get past. This was done on a PvP server, but would work after the Tol Barad battle on a PvE server, seeing as everyone coming from the bridge will be flagged. I did this on a frost mage but there are obviously ways to do it with all classes.
My basic strategy is to lie in wait at the entrance to the bridge. When someone comes past freeze them in place and blink to the guards. The guards then kill whoever I froze. Note: they do not have to attack me at all. I can be 100% the aggressor and the guards will still kill them.
The method I had best results with was pet nova them, sheep them to dismount then run to the guards. If they trinket, as I am running back to them with the guards I will fingers of frost deep freeze them so they can't get away.
I've also sheeped people out of invisibility if I saw them coming, blinked to the guards then pet novaed them as I come back so they can't get away. There's a bunch of ways to do it.
http://i1221.photobucket.com/albums/...o-guards-0.jpg
http://i1221.photobucket.com/albums/...-to-guards.jpg
http://i1221.photobucket.com/albums/...-to-guards.jpg
With practice this constitutes an almost total lockdown of the bridge. Best used directly after the Tol Barad battle has been won by the opposite faction.
Once you know what you're doing there's not a hell of a lot people can do to not get killed. Basically I have to miss them because I'm getting someone else killed and even then there are ways to get multiple people killed at the same time lol.
Additionally, if you attack someone while the guards are out.. and go near the guards while they are out they will go after the person you attacked. This means you can get people far from the guards posts killed. Here I get someone just watching from next to their guards killed by attacking them when the guards were chasing someone else:
http://i1221.photobucket.com/albums/...-bystander.jpg
I've made the guards fight a number of times. It's 3vs2, so the Horde guards win, allowing for a small window within which you can actually kill people at the Alliance guard post, like I do to a hunter here:
http://i1221.photobucket.com/albums/...vs2-guards.jpg
You can have a look at more results of me doing this over and over again at the photobucket:
Pictures by cataclymsguardsarecool - Photobucket
There are plenty of ways for other classes to accomplish the same thing:
Warlock : Deathcoil, fear curse of exhaustion someone and use demonic portal to port get near the guards
Hunter : Sit there in camo, when someone comes past scatter trap them and disengage to the guards
Death Knight : Simply death grip and chains of ice them, then run to the guards
Warrior : Charge them, hamstring, then heroic leap to the guards
Rogue : Sit there in stealth, shiv on cripping poison, stunlock or blind and sprint to guards
This might constitute a zone disruption if done REALLY persistently, but basically they've made the guards this way. If this is an exploit so is running to the guards at one of the bases if you're getting ganked because this same mechanism works there too. Same with running into cities or towns in general, those guards won't two shot someone, but it's the same thing. I've been doing it A LOT and haven't heard a peep anyway.