Let me try to explain this again...
You can hit the mobs even when they're at like 5% or what have you, and you'll still get credit. (Go try it out, really. These aren't like your normal average quests.)
Now for the tagging part I tried to explain, but apparently failed to get across.
As I tried to explain, Blizzard made you get credit from guard kills and other players helping because they didn't want this to be a pain in the ass.
I also tried explaining, that, the reason you can drop combat is because the MOB was TAGGED by you. When does a tag reset? When they drop combat, or you get WAY the **** away. They never drop combat because of the guards and other players around (but mainly the guards) so their tag does not reset. This is also why I tried telling you to get on a second account so you can see this for yourself.
Show me getting credit when you're in ORGRIMMAR and I will believe this is an exploit. Why? Because this is no different than being grouped with a player doing the killing for you. If you're like a 100 yards away and a player kills something, you still get credit. Who the hell cares that this happens to be a guard. They intended you to get credit for doing almost no work.
You can also go to Sethria's Roost, white swing punch a mob for about 1,000, stand there AFK, get out of combat, and let other players finish it for you. You will still get credit because this was INTENDED.
So let's combine all of this:
1) Blizzard wants you to get credit for doing almost no work (tiny hit of ~1k is enough)
2) You tag a mob that is in combat, so it does not untag
3) If you chill away from another player within 100 yards, you get credit. Why wouldn't you if a guard kills the same said quest mob? This is not like guards/other quests where you do not get credit if a guard touches a mob, or you did not do enough damage.
4) The quest credit range is still within the bounds of normal quest range credit.
So would you consider the following an exploit?
1) You AFK at the Molten Front Portal while a buddy kills/quests down in the fields and you get full credit, out of combat, far away.
2) You tag a mob, another player attacks it, you drop combat and leave it on them for them to kill. You get loot, credit, everything.
No? I didn't think so. Get past the goddamn "omg it's a guard" part already.
If you still don't understand this, I give this. I don't know how to explain it any dumber. If you have a legitimate question however, I'll answer. You guys just seem stuck on the whole "OMG THIS IS NEW AND EXCITING, A GUARD HELPED ME AND DIDN'T SCREW ME OVER!".