i'm sure someone has posted already but i didn't read anything so here is the reason why:
if you have 10 bil$'s that you need to hold somewhere... who are you going to trust at 0% rates? jpmorgan? goldman sachs? mf global? that's why they keep it with the government at those rates. for big hedge funds there aren't any secured facilities they can deposit with, so by default they have to keep it in tbills regardless of the rates as it isn't feasible to keep those sums in benjamins and redeem them when you need to.
if you have 10 bil$'s that you need to hold somewhere... who are you going to trust at 0% rates? jpmorgan? goldman sachs? mf global? that's why they keep it with the government at those rates. for big hedge funds there aren't any secured facilities they can deposit with, so by default they have to keep it in tbills regardless of the rates as it isn't feasible to keep those sums in benjamins and redeem them when you need to.