One of the things I like most about print mags and subscriptions is that I can keep them and look back at things printed in the past to compare to new items. Back issues are also great for reading material on really boring days. If you have a Kindle and you have a digital magazine subscription, you may not know this little tidbit.
If you cancel your subscription to a magazine on the eReader, you also lose your back issues. That is like the publisher coming to your house and picking up the magazines on your coffee table if you cancel. That is certainly not fair at all to the subscriber that paid for the issues.
Some people like to collect the back issues and it really sucks that you lose those issues. Not only do the back issues you already downloaded get deleted if you cancel a subscription, but there is no way to transfer back issues to a new device either.