It's not a reason for being idle >ok . That is not a good reason for (you) being idle ( you answered them when they reply to you with an answer that you cannot accept)).
If you see them idling > You don't have to idle ( even if your work is finished ) . Go find something else to do
If they answer you with a good reason > That is still not a good reason why you should idle
You should use 'good reason' because their reply is telling you the reason . So, is it good or bad? Unless , their reply is totally unacceptable