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?
I'm sure the person you're thinking of is the one I had in mind when making my comments.