I think t-bonham was saying that they’re not all in prime space. At my mega mart, each checkout line has a few magazines and then off to the side they have a huge rack with probably a hundred other ones. That huge rack is essentially dead space, at least in my store. It’s near the checkout, but you have to be looking for it, you have to say 'hold on, I want to grab a Men’s Health/Guns & Ammo", there’s no impulse stuff over there. It’s in the area between the very first checkout and the wall separating the rest of the store. I think most Targets fill this area up with batteries and random kids stuff (Pokemon cards etc).
I regularly purchase certain magazines from the grocery store/supermarket/whatever you want to call it. For example, I usually espy the new Cook’s Illustrated there, check it out to see if I want to buy it, and, if I do, see if there are ingredients I’m going to want to have in my cart for use in the interesting recipes it has.
As a general rule: a grocery store has a very thin profit margin; they do not waste space on things that don’t sell, though there may be some limited space assigned to things that don’t sell quickly, but which are offered as a service to keep you happy.
Unlike just about everything else in the grocery store, magazines are a two sided market which allows them to sell at a smaller margin for the publisher.
I worked at a grocery store cash register for a just a little bit in college. The single biggest thing I learned there was that everything there gets bought by somebody at a fairly constant rate. All the time, people had whole carts full of stuff that not only did I never buy, I never dreamed anyone would ever buy and I had never seen bought. And I guess my grocery cart would seem just as alien to many others. People are more different at home than they are out in the world.