I agree that it sucks when teammates kill you. On the other hand, I'm not sure it's ND's responsibility to make sure jerks can't be jerks. Some people just suck.
And to give a counterpoint: A while ago, I had a teammate kill me with quick-boom so he could get the 3-medal pickup I was going after. I know it was immature, but it sure felt good when I blasted him with the RPG later in the match. Sometimes you can use team killing to make a point.
ND put team killing in this game though, and I really don't understand why.
The reason TK is allowed is because ND wants you to be smart with your grenades. I honestly hate it but imagine no TK. You're going against this guy when his teammate throws a grenade. With TK, if you both get killed by the grenade then the guy attacking you won't get a kill but you will since he died. If there wasn't. Well... no kill for you. It's ND way of saying, don't be throwing your grenades around like there's no tomorrow.
Teammates can't throw back team nades.
... Which is another reason why you shouldn't be able to kill teammates with grenades. They can't throw them back, increasing the chances of dying DUE TO A TEAMMATE.