Because what is the point if something is distributed in a restrictive license, can't be preserved and then gets lost to time? Also, licensing is to avoid distribution, modification or outright copying by competitors; preservation is completely orthogonal to those concerns. It is to avoid losing a piece of craft to the sands of time. There is no reason laws should have power over anything in perpetuity.
As seen in other spaces, pirates ignoring the "law" will provide the greatest service to humanity.
If the rights holders of the licensed bits haven't abandoned them, then it's not really fair to distribute them without their consent.
Also, we're not talking about personal code either, but something that is arguably a product humanity, or a part thereof, would want to preserve for posterity.
Lastly, no one is telling you to open anything. I am saying that if someone decides something you have created need to be preserved, they should go ahead. You can protest, you can sue, the point is it shouldn't stop anyone from trying. Which doesn't apply to this case, as the owner of Winamp actually wanted to make it open source.