Probably because Virgin operates in a far more competitive employment market than SQ does, and its employees are protected by stronger employment legislation.
As for SQ's investment, they bought at the top of the market, with their eyes open. Any airline investment at that time would have produced a loss now (think how bad it would have been had SQ invested in a US airline!), but Virgin - profitable last year, unlike many, and likely to be profitable in 03/04 despite a shortened financial year - is more likely to produce a return for SQ than almost any other airline investment would have done.
The SQ guys on the Virgin board seem quite happy, by all accounts!