In my point, if a normal person visit a site whether it is ISC or Quora will not go to search any of the answer whether it is copied from anywhere else! The main aim for the person will to get the answer he /she is looking for! Once it is done, its done.
Secondly, how many people go to any site with this mind set to know whether the contents are copied or not? Very few, mainly the author who are actives in the online writing. Which are very few in numbers comparing those who are looking for answer.
Thirdly, almost everything is looking after by Google nowadays, then what they are doing? If the site has copied content, they must close it or warn it.
Fourthly, even if monitoring by authorised people in ISC, still here or there found copied content. Whether it is in ask expert or Forum. It simply mean that monitoring is not an easy job.
Comparing all these, I would only say that the Quora should take action on the authors those who copy and paste or teach them like how ISC does with their members. If not Google should take action to shut that site.