Is it ethical to keep authorized readings after course is over?
Technically, it may not be ethical to keep a case or data set past its license period. Most dataset licenses also specifically state that the holder should not hold material of both physical or digital form (or even preprocessed form) of the licensed matter past its license term. It also stated that the backup of all forms should be deleted. 

Having stated above, it is still a widely practised taboo to use cases and datasets past their license periods. You may attain knowledge from it but will only get into trouble if you were to publish any material that relates to the dataset/case that you currently possess beyond its license. This also depends on how strictly the license owner enforces the license.