There may be an asymmetry, but there's also a matter of choice and control. You can (at least presumably) control whether you will break a contract (NDA or oth

There may be an asymmetry, but there's also a matter of choice and control. You can (at least presumably) control whether you will break a contract (NDA or otherwise) in the future. You cannot control something that has happened in the past, and thus you have much less choice over your actions if you're blackmailed.