As far as I know they were related to some scams by using a website called paidbd. Besides, they used to play bet. Being cheated from them someone reported about them to the police.
If this is true then they where arrested based on their crime committed and not because they used Bitcoin. Criminal art is criminal art and when its be committed while trying to secure Bitcoin illegally, then its a crime to humanity and is punishable under any criminal law in the state. Let them face the music.