But the NEM group has pledged never to rewrite blockchain records, so even those "transactions" resulting from a hack will remain valid.
These hacks force interesting decisions to be made, like when ethereum forked. Like would users want a fork here, or to have the records rewritten, or would they like for NEM to not control the blockchain. If they can rewrite it for a hack someone could potentially rewrite it for some other reason without telling people.
It opens the door for things we may not want to see on the blockchain and removes one of the things that people support about the blockchain: a true record. It's interesting because no one wants to be hacked, that's why security is so important.