They are not designated as terrorists by Bangladesh. The US terms them as "other groups of concern".
http://www.state.gov/documents/organization/65479.pdf
And since they are not a threat to Bangladesh's national security, its security forces are largely indifferent to the matter.
It's not easy to monitor the porous borders. Can I also blame Indian forces for letting them slip into Bangladesh? India shares the largest border with Bangladesh among all neighbors, not China or Pakistan.
And finally, publicizing these findings is just as laughable considering today's communication technologies (i.e. those guys living in huts can simply mobilize after receiving this 'news'.)
I find it hard to believe that people actually bought this garbage.