Could it be that they want to sell the magazine to subscribers that aren't willing to wait for a particular issue at home?
Take for example Wine Spectator. It comes out on the newstand with the best 100. If you wait for the copy to arrive at home, the best 50 are sold out and you are stuck with the next best 50. Of course you are a real sucker if you subscribe to this magazine, because they sell subscriptions to their web site where the information shows up first. These subscriptions are password protected. Guess what, a lot of those passwords get shared, so as a magazine subscriber you are paying for late information that others get early for free.