Magazine Inserts: Missed Marketing Opportunity

I subscribe to a few magazines and come across dozens more every month. One thing I've noticed is that there seems to be a major missed opportunity with magazine inserts: marketing to subscribers.

When you subscribe to a magazine, you get the same blow-in inserts as the newsstand version. Why? Wouldn't it make more sense to, perhaps, thank the subscriber for reading the magazine and give him a value added bonus? Or a tailor-made message exclusively for subscribers? Just something that separates the subscriber base from the newsstand base, and gives a more personal, rewarding quality for the most loyal readers.

My thinking is that subscribers shouldn't be subjected to continuous "Subscribe Now!" postcards when they already do receive the magazine every month. Printing those kinds of cards are pestering to subscribers and generally just a waste of paper. And a missed marketing opportunity for the magazine publishers.