Is it ethical to keep authorized readings after course is over? I was the TA for a course a few years ago.
As part of the course, I was able to obtain a set of business case studies.
At the bottom of each page, it is written something to the effect of:


  This document is authorized for use by [my name], from [start date] to [end date], in the course [name of course] at [name of school]. Any unauthorized use or reproduction of this document is strictly prohibited.


Is it ethical for me to keep these readings after the course is over,
and to read them some time in the future?

Conclusion

I appreciate the opinions of all who answered and commented.

I had asked this question because the licensing agreement
sounds too bad to be true.
The students had to pay more than $100 for all the readings for this course.
If the students were to purchase a textbook,
at least they are allowed to keep it and read it in the future.
I suspect that the publishers wishes they had the technology available
to make the case studies like the
"Mission Impossible" style self-destruct messages
which would explode once it is past the license end date.
This seems acceptable to me for software,
but strange for something like readings.

At the end of the day, I have to make my own decision
about what is the right thing to do in this instance.
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.