A director's statement is on the film's Facebook page.
The story of the ban is getting widespread coverage, with articles by the Bangkok Post, The Hollywood Reporter, Film Business Asia and the Associated Press.
According to the Bangkok Post, the censors feared the film could "'persuade viewers to falsely believe' incorrect information."
Film Business Asia carries the official statement from the Culture Ministry's censors:
"The Film and Video sub-committee do not permit the documentary film Boundary (Fah Tum Pandin Soong) to be screened in Kingdom of Thailand. The film's content is a threat to national security and international relations. The film presents some information on incidents that are still being deliberated by the Thai court and that have not yet been officially concluded."
According to the Bangkok Post, the censors had issues with "groundless" points made by documentary, particularly a text caption early in the film that sets the stage for the film's political context, explaining that there were "nearly 100 deaths" during the government's crackdown on the red-shirt political rallies in Bangkok in 2010. The government insists the official figure is 89.
Censors also took exception to a long monologue by a Cambodian soldier who criticizes Thailand.
On the ongoing Thai court proceedings, I am guessing the censors are referring to the cases regarding the red-shirt crackdown.
The border dispute is being debated at the moment by the International Court of Justice, with the Thai legal team recently celebrating its return to Thailand and possible success.
Boundary premiered at this year's Berlin International Film Festival. It has also already screened for festival audiences in Thailand, at the Salaya International Documentary Film Festival, where it was the opening film and screened once more.
Nontawat had submitted Boundary to the censors in hopes of obtaining a rating so that it could be screened in a general theatrical release.
He tells The Hollywood Reporter:
"I really didn't expect this film to be banned. Everyone I’ve spoken with who’s seen it says the film shows the point of view of every side, and that the film is neutral. My intention was to let the film be a space for the people in the troubled territories to voice their views and feelings to the outside world, which they haven’t had a chance to express in other Thai media."
With Boundary, three films have been banned under Thailand's film classification system enacted in 2009. The other two are Tanwarin Sukkhapisit's transgender fatherhood drama Insects in the Backyard and Ing K. and Manit Sriwanichpoom's political satire Shakespeare Must Die.
Thailand has a six-tiered film ratings system – G for general audiences, P for "promote" as educational, 13+, 15+ and 18+ suggested viewing ages and the restricted 20- rating, which requires ID checks. "Ban" is the hidden seventh tier of the system.
Nontawat says he'll appeal the censorship board's decision, but isn't optimistic of the outcome.