They would only lose the block, if it was profitable to do so. If they win the bet,  they would face only a tiny extra cost of losing a block (likely slightly slower block propagation, due to mining a few extra transactions that the network wouldn't have seen)

And keep in mind that for a shady pool to withhold a valid block might not actually cost them very much at all. This is because they can cheat their miners, and just act like it was bad luck that caused them to have not mined a block in a while.

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(Although in practice it's not really a concern. For someone to exploit them, they would likely have to risk 15 bitcoin (With the hope of winning 1). I doubt anyone is ever going to do that, as the chance of them not getting paid would be too high)