~snip~
When you read articles from a shitty source, then your conclusions are just as shitty (excuse the expression). As @stompix already wrote, it is not about selling 50K BTC, but only a part of what has seized two years ago. And the fact that they (the US government) sell BTC means that they informally recognize it as something that has value, and you could only worry if they destroyed all the seized BTC and thus showed the middle finger to everyone else.
What you see as a problem is actually a good thing, because the US government promotes Bitcoin by doing so and also puts it back into circulation - and in return it gets what it loves most, its dollars.