This question is not related to drupal, so I tried to close the question but it not allowing me to do that. Why not to close the question which is of-topic ?
This is by design: Questions with a bounty cannot be voted to be closed. The reason is that closing a question with a bounty is an exception to what normally happens, and it needs an exception hander, which is invoked by flagging the question for moderation attention.
It is true that users could use bounty to circumvent the question closure; if that is the case, a moderator intervention is still necessary to investigate what happens, and see if the user keep using the bounties for that purpose.