If a pool discards S.DICE transactions but intentionally allows competing transactions from same source to process then the pool only needs a few % of network power for anyone to profitably attack S.DICE by trying to double-spend any losing bets - the few % of times the pool includes the double-spend (and cancels the losing bet) then outweigh the house edge on the rest of bets. You underestimate the extent to which some are opposed to S.DICE (not me - obviously).
I don't know if you're aware, but there's already a bot that routinely double-spends lots of losing SatoshiDice bets. Its transactions almost never get picked up by the miners though. All it would need would be for an anti-SD pool to start deliberately mining the double-spent transactions to make SD unprofitable. Can you think of anyone with a mining pool who's vocally against SD and has used the pool for morally questionable things in the past?
Roughly 3/4. Perhaps the whales weren't quite such a big part of S.DICE profitability as everyone thought?
Or the whale lost so big in the first 5 days of February that he decided to stop playing, but gave the site enough profits from his run of bad luck to almost make up for his absence the rest of the month.