I don't think it's punishment, it's a deterrent. The point isn't whether the government knows it. The wife didn't know it. The blackmail wouldn't be "we'll tell

I don't think it's punishment, it's a deterrent. The point isn't whether the government knows it. The wife didn't know it. The blackmail wouldn't be "we'll tell the government you're having an affair", it's "we'll tell your wife". It's only "no longer any security threat" because he got caught. The punishment is to dissuade anyone else from having an affair, which a foreign government could use to make their lives difficult by telling their spouse.