If the US government wanted to take bitcoin down, why waste $30mill doing so. They can just declare all aspects of bitcoin usage / trade illegal with severe jailtime. With their influence they could then convince all other western institutionalized countries to do the same thus removing the value of the currency.
Much easier than actually "destroying" it - I mean what's the point of that. People using it for illegal activities will just create another coin. or jump from Bitcoin to Litecoin to Dogecoin etc..
I think they're working out a way to make $30 mill from it rather than spend $30 mill destroying it

besides, how many coins did the FBI confiscate from Silkroad? why would they want to throw that value out the door? !