They could belong to anyone. All that person(s) has to do if file a claim and prove ownership.
Obviously no one including Ulbright himself attempted to claim them. Therefore, no claim of ownership so it goes to auction. There is a risk involved with claiming property that was involved with illegal activity. At no point were the owners of these bitcoins ever held back or hindered from officially claiming them. If you were doing nothing illegal, there is very little reason not to claim ownership.
This is standard procedure and has been this way for decades. No different then the local police confiscating stolen car stereos and iPads. If no one claims them, they are auctioned off.