"Between you and me" Violating Privacy with Impunity

I noticed this entry by Greg Popcack on his right wing HMS Blog in which he fires of a letter to Joe Feueherd of the National Catholic Reporter criticizing his article last week.

Joe replies and send him an email that begins with:

Thanks for your note. And, between you and me, I agree that it wasn’t my best work.

So, of course, what does Mr Popcack do? He plasters this private email on his website. I'm not sure what part of "between you and me" is that vague. Even between idealogical opponents, some courtesies need to be established.