Usually one has to wait for large dividend which the bucketeer cannot cover from his profits, and at that moment they're exposed.
The real bucket shops can cover any amount of dividends because for every long position there is a corresponding short position. When someone is receiving dividend, other is paying dividend. For example when John has 100 long shares, Thomas has 100 short shares. Of course there is exceptions but statistically the profit from non-hedged big positions will be almost equal to the losses from non-hedged big positions.
The wise bucket shops will hedge too big positions that cannot be hedged within the bucket shop.