At a guess it's to sandbox the Steam ecosystem? There was a Steam bug in the past where it would delete all contents of the user's home folder. I guess also Steam and its games are all closed source and people might not trust them as much.
It's very interesting that he asked the media to send coins to one of Satoshi's Bitcoin addresses, and that he would return them. All of Satoshi's original coins are Pay-to-Public-Key, whereas the BBC's money was sent Pay-to-Public-Key-Hash (P2PKH). This means that Craig would have to present a signature along with a public key that just has the same hash as the Satoshi public key, not a signature for Satoshi's public key itself. I think he is banking on finding another key with the same hash, as a final desperate attempt to forge his identity.
The use of & instead of && here means that openssl start running at the same time as the base64 decode command. This means that openssl could start reading from an old version of the signature file before base64 starts writing to the new one.