Certainly; but they have to do so legally. There is a limit on the number of Bangladeshis that can migrate when done in LEGAL terms regardless of whether he follows an organized religion or a way of life just like number of immigrants to UK and US is limited by some numbers. But due to the illegal mass influx, this creates hatred among local communities.
The reason why BD Hindus/Buddhists were asked to be granted Indian nationality is that because they complained of discrimination and target attacks in BD during the early days of BNP. This is not because of Bangladeshi government since they are friendly to us but because of the narrow-minded thinking of certain "community leaders" who try to use such stunts to gain political mileage. The Hindus/Buddhists in question, complained that they were threatened as "Indian kafir spy" and other derogatory terms, despite having proven records of being born and raised in Bangladesh as any other Bangladeshi is.
Hence, the proposal was made.