It is a mechanic put into the game to stop people breaking the game by killing important quest givers by mistake. I agree it is a little annoying, but if they hadnt, many people would no doubt be complaining that alot of the quests did not work. People on the PC can change this via mods or consule commands, sadly not so easy for Xbox or PS3 players. Hopefully the Devs may adjust this in a future point, as it can be a tad silly....
Heaven forbid we should have to deal with the consequences of our actions...

This is one of the reasons I've put doing the Civil War quest line off until I have the CK and can change the 'Essential' tag to the one used by your companions, so that at least I, personally, can kill them. In fact, I'll be doing that to every essential character in the game, and if that means I break quests by killing people,
so be it. I certainly wont come on here and complain about it, since I knowingly did it to myself and can handle the results.