The only reason why the Dalai Lama or any body else does not sue NKT or Kelsang is that they can — unlike NKT — bear that there are different opinions about them, even if it is untrue or defamation.
It is a sign of cult leaders to threaten to sue ex-members or critics because they are full of themselves, they think they are godlike or Buddhalike.
Because Kelsang Gyatso is a cult leader and NKT is a cult they threaten forums, academics and individuals to sue them.
Because the Dalai Lama is not a cult leader but a relaxed and open minded person, it is no problem for him if he is slandered, he just will not take any lawyer to defend himself. He is also used to be defamed by China's leadership and press and other hostile human beings. He takes it with ease and humour, because—unlike cult leaders—he has no exaggerated feeling of self-importance as cult leaders have.
Hence what the Dalai Lama says about the Western Shugden Society expresses his qualities: "I am extremely happy that they can enjoy freedom of expression."
No lawsuit threat, no lawyer, only peace"