So screw over all the people that made transactions with the hacked bitcoins before the hack was found.
Holy fucking shit! Now that I think about this, it is a massive timebomb waiting to happen! Once you have hacked the bitcoins, you can tumble them with other bitcoins, exchange them to other cryptocurrencies (which are also 1 way transactions so the person exchanging with the hacked coins is completely SOL), or exchange to fiat currencies (although that would probably be the most traceable so probably not). As you tumble and exchange the cryptocurrencies more and more, it becomes more and more difficult to hard fork the currencies to undo the hack.
My wild prediction. At some point:
1: Some black hat will gain a controlling compute share of a cryptocurrency (either the chinese ASIC companies or a massive AWS/google cloud/azure... cloud mining to override the majority share and convince the network that some illegal transactions occurred).
2: They will then proceed to massively tumble the hacked coin through different exchanges through multiple different cryptocurrencies so that hard forking is nigh impossible to undo the hack.
3: Cryptocurrencies all crash because they are susceptible to mining takeovers which cannot be detected quickly enough to hardfork to undo the hack which render their whole design impractical.