I doubt it is illegal. It seems like it is in the same vein as those who file nuisance lawsuits against the activists to tie up their resources so they can't fight the real problem. An dto send a clear message to other who might think of filing in the future.
I would question just how "accidental" the leak was as the implied message is that if you actively support the point of view of those on the list, you too, can find yourself on a list. Hopefully it was verified before it was distributed?
And to link it to music
As some day it may happen that a victim must be found, I?ve got a little list--I?ve got a little list The Mikado