Why does US government plan and need to sell 41 thousand Bitcoins this year?

So basically US government has above 200 thousands Bitcoins. They have them from Silk Road seizure (70k), Bitfinex (96k), Silk road hacker seizure (51k) etc....

They have announced that they're planning to sell 41 thousand of those during this year. They already sold quarter of that about a month ago, so they will probably do the same thing 3 times more during this year. By current BTC value, 41k bitcoins is about 1.2 billion USD. That's a drop in the ocean for them, why do they need to do it, and how do you think that will affect Bitcoin in future, especially if one day they decide to just dump the rest 170k+ Bitcoins.