What you say is true. But if a technique makes it so that 1) they don't preserve a record for the future, and 2) they do leak (or risk leaking) information that can kill service people, I personally care more about #2.
(Ironic that, in trying to not leak to future investigators/prosecutors, they increased the risk of leaking to foreign adversaries. Shows which threat they're focused on.)
(I'm not defending the Trump administration's law-keeping in general. I'm asking about this specific set of communications.)