My understanding is that it is the index managers not the fund managers. Indicies started just as nice ways to track the market or sectors in one number (which is why in reporting and checking on the market you will see those numbers quoted). When an index fund decides to track an index and define and market themselves as doing it, the fund is legally obligated to follow the index. Several analysts at the FT has been complaining about how much power this gives the people in charge of the indicies and that most people haven't really caught on to.

Edit: to make it clear guys at something like Standards and Poor (S&P) decide on what to include in the index and the rules around it and then someone at a company like Blackrock or Vanguard markets an index fund that will track that index to investors. So in this hypothetical, it is S&P screwing over Vanguard investors.