From my understanding: to pose the problem for miners, you hash the block you're planning to submit (which includes all the proposed transactions). Miners only

From my understanding: to pose the problem for miners, you hash the block you're planning to submit (which includes all the proposed transactions). Miners only get the hash. To claim the reward, you need the preimage (i.e. block to be submitted), which miners don't have. In theory, you could watch the transactions being broadcast, and guess (+confirm) the corresponding block, but that would require you to see all the transactions the pool owner did, and put them in the same order (the possibilities of which scale exponentially with the number of transactions). There may be some other randomness you can insert into a block too -- someone else might know this. Edit: oops, I forgot: the block also contains the address that the fees should be sent to. So even if you "stole" your solution and broadcast it with the block, the fee is still going to the pool owner. That's a bigger deal.