Because it's the truth!
It's not the truth. Lee refers to the Constitution by name in the letter.
Walt
So. Here's the text again,
'The framers of our Constitution never exhausted so much labor, wisdom and forebearance in its formation, and surrounded it with so many guards and securities, if it was to be broken by every member of the Confederacy at will. It was intended for "perpetual union" so expressed in the preamble, [contained in the Preamble of the Articles, not the Constitution] and for the establishment of a government, not a compact, which can only be dissolved by revolution, or the consent of all the people in convention assembled [contained in Article XIII of the Articles, not the Constitution]. It is idle to talk of secession [from the Articles, which prohibited it, not from the Constitution, which reserve the power to the states].'