Well, here's a rather simplified explanation: The samples were a promotion to boost newsstand sales and (we hope) convert one-time sales into new subscribers. Unfortunately our post office permit is base on mailing magazines--only magazines. It is cost-prohibitive to add anything that does not qualify as editorial material. Furthermore, the magazine is bundled with other magazine titles when it is subscription-mailed, and such items would interfere with that process.
Bottom line: It's cost-effective to include such items only on the newsstand, and increased sales help pay the costs.
I'm sorry we're not able to add the items to subscription mailings, but the cost-savings from being able to use postal magazine mailing rates benefits our readers in terms of keeping costs down. Thanks for asking