Curious about this. Perhaps my understanding of a 'block withholding attack' is inaccurate, but are you saying (bolded above) that if a block withholder finds a block they can keep the 25 BTC reward for themselves?
No, they can't keep the 25 BTC from blocks they find. They can send the block to the pool or throw it away. Only those two options.
so the 25 BTC are lost.
So what is the incentive here?
Is this like a goldfinger-tactic? Trying to increase the value of the bitcoins you already have by destroying other bitcoins?
Like the goldfinger guy from the james bond movie, who wanted to increase the value of his gold holdings by radioactively contaminating fort knox.