According to the article "tens of thousands" of advance copies are sent out. So if you want the reward per leak to be $10, the cost would be hundreds of thousands of dollars. Maybe that's worth it to a studio, but it definitely is a significant junk of money.

Studio pays out that amount in case every single receiver of advance copy leaks it, is it the case? I would guess that there are only a few who leak it. Also, I think, studio can start legal action against the leakers, so it could collect fines in case of leak.

The problem is that they must pay the bounty for every leak in advance! (As I said, they could redeem unclaimed bounties after some time, but they must put up the full amount in advance, and are thus vulnerable to exchange rate fluctuations)