Surely you remember that not so long ago, one member was caught with his 2 alt accounts due to the public data of the campaign - and in the event that this data was visible only to the manager, he might still be abusing it today.
Correct me if I'm wrong, but this link was made purely on the blockchain history of the payment addresses. Making such a link would still have been possible without the spreadsheet simply by pulling the payment addresses from the weekly payment transaction. The investigator might not have known the exact users linked to those addresses, but could easily just pass the blockchain evidence to DarkStar_ who could then respond accordingly.
I see no reason for there to be a public record linking usernames to addresses.