Of course they are. They can do what they want.
Despite criticisms to the contrary, there is no injunction within the New Kadampa Tradition (NKT) — in the Internal Rules, the NKT books, by word of mouth, or elsewhere — against students reading non-NKT books. Geshe Kelsang’s commentaries to Je Tsongkhapa’s teachings form the basis of the three NKT study programs (the General Program, Foundation Program and Teacher Training Program, see Internal Rule 15); but outside of their studies NKT students can and do read whatever they wish. Everyone has freedom to do as they choose.
One statement has been taken out of context and exaggerated on various chat groups and forums. Thirty-five years ago an NKT Resident Teacher taught his students in a class: “You should not read books other than Geshe Kelsang’s books.” This teacher’s motivation was to help the students to pass their examination by preventing distractions arising from reading other books. However, this has never been an NKT rule or even suggestion.
I personally witnessed book by other teachers being removed, teachers removed from teaching because of quoting from texts by phabongkha and warnings against mixing traditions.
While it is allowed, it is certainly discouraged, not least of all because it is expected that any sincere student of Gyatso will become a teacher to pass on the blessings, and this must be done purely to prevent degeneration.
The author either knows the above and is disingenuously leaving the above out to claim something that is technically true but misleading due to lack of context, to avoid actually addressing the complaint while seeming to; or they are unaware of things that are common place and both written and unwritten rules of teacher training program and foundation program studies.
I am assuming this was some years ago. I have been practicing and teaching in the NKT for over 40 years and I have always had other Buddhist books as “light reading” — no one has ever told me not to and I wouldn’t listen if they did. I have never felt discouraged in reading what I want. I haven’t met anyone else in the tradition in a number of years who feels that this is an issue either.
I stick to Venerable Geshe-la’s books as the basis of my daily study and meditation practice because to me they are incredible. Likewise, Tharpa sells Ven Geshe-la’s books as the core of the three study programs. However, you can get other Buddhist books and magazines from any decent bookstore and many of my Kadampa friends do.
Plus, Je Pabongkhapa is one of our lineage Gurus, I have his books right here on my shelf too.
Many years ago an NKT Resident Teacher did say to his students in a class (and I was there): “You should not read books other than Geshe Kelsang’s books.” This advice was unskillful and has been quoted ad infinitum, but I suspect this teacher’s main motivation was to help us stay focused and pass our exam. However, other than this, in my 4 decades in this tradition, I can say that this has never been an NKT rule or even suggestion. Ask pretty much any practitioner or look at their bookshelves or browsing history, and you’ll see that they don’t only read NKT books.