People who know someone is running it down yet still take the kills should be punished. They are just as bad as the inter for contributing to this problem.
No, they shouldn't, and no, they aren't. They're still bad to some degree, but not for the reason you stated.
They're bad because of employing underhanded tactics/taking underhanded opportunities to win the game. Not because they're "contributing to the griefer".
If people stop taking kills from raging intentional feeders, it won't happen anymore because the player wouldn't get what they want.
That's not how it would work at all. If someone starts running down lane, they're doing it exclusively to fuck over their team, and regardless of whether or not someone tags them and claims the kill, they're still going to achieve their goal by making the match a 4v5. If people stop taking kills from raging intentional feeders, the feeders are still going to run down mid, because what they want is to ruin their team's chances of success.
It doesn't matter if they get executed by minions/turrets or killed by enemy Champions, their goal is simply to throw their Champion down a lane and watch them die.
It's fine to frown on underhanded gameplay, but don't equate it to being the same as trolling.