Here is the game that magazine publishers play: they frequently send renewal notices that make it difficult to determine when the current expiration date really is. It appears that when Mom received such notices, she would send them checks under the mistaken assumption that the publishers were being honest and that her subscriptions were actually about to run out.
Moreover, the publishers have access to demographics information and know that subscribers who are age 80 or higher are likely to have some degree of mental impairment -- their memory, their record-keeping, etc. Effectively these publishers are preying on the elderly.
It's not illegal, but I think it's immoral.
To make things worse, getting a refund when canceling a prepaid subscription isn't easy. Some websites are more difficult than others; it took me a while to find out how to cancel Vanity Fair even though I'm pretty good with websites. And when the publishers calculate the refund due, they pro-rate the issues already received at full price. Of course, the renewals were at deeply discounted rates. The consequence is that refund amounts are tiny.
If you have elderly family or friends, beware of this scam.