This doesn't have to involve the reversal of all those transactions but could be used to identify those responsible and those that empowered them for restitution.
Good points about contracts vs. regulation. A hard fork would also be possible invalidating the BTC which were stolen. Common law holds that what you steal is not yours to give or sell.
Assuming the theft was large enough and the mixing extensive enough, this would interestingly have the effect of probably decreasing everyone's BTC by some small amount.