Nobody knows who transfer bitcoin to whom as it is anonymous so government can't force anyone to pay tax for bitcoin related transactions. So simply saying no bitcoin transactions aren't subjected to tax or can be taxed.
For now bitcoin still not government involve and i think its impossible that bitcoin will be involve in government unless if we force as to use bitcoin to accept it locally goverment will seen it if it will popular in our country. you know governments are corrupt..
If bitcoin steadily grown be a famous of course the government will think about how they can get tax from bitcoin because bitcoin transaction at this time was amazing.