These flights are charter flights, so the service level is decided by the customer (Slush). If Finnair decided to go beyond that and pay for it from its own marketing budget, other customers would start to expect the same in the future and try to get away with paying less.
Obviously, there's some minimum service level that Finnair finds acceptable, but below that point they simply won't do business with you at all.
Also, if there are SFO-based Oneworld emeralds on this flight, they will have flown enough to understand that it's a charter flight with special rules. Likewise, anyone buying a regular ticket and leaving from HEL will know what they're getting into. As long as Finnair demanded that proper food be served, everything will be fine (edit: or duct taped).