"Granted, there are also letters of condemnation, but not enough to satisfy my sense of outrage"
You're referring to the editor section of the NYT? If so I would imagine its because they put most of them in the "evil american" basket.
Yes, I would imagine the Slimes has no scruples whatsoever about printing the letters that show them in the best light.
They are aware of their circulation numbers, make no mistake about it.