If it would be priced in terms of Bitcoin it would
make the bounty less attractive for a lot of people.
Bitcoin is highly volatile and not everyone believes it will rise.
15 XAU is by the way currently around USD 18000.
15 XAU is by the way currently around USD 18000.
The problem with setting the price in Bitcoin is the volatility: someone with information could attempt to time their contact with Theymos to maximize their bounty. This could significantly delay revealing the information, if the bounty claimant felt the price were temporarily in a slump, and could cost Theymos a lot more than expected if they got lucky.