As a former book selling drone (for Crown books which was then sold to the evil hive-mind known as Books-a-Million), i can fill in a few pieces of info here that are missing.
First of all, Borders owns Walden books, so if your local Borders sells gun rags, but the Walden does not, it has nothing to do with corporate policy.
Secondly, Walden books sucks, why were you even there? Walden books tend to have a pathetic selection of books and mags, and what they do sell tends to be stuff that sells. Last time I checked, pot and female breats tend to be very popular, which drives magazine sales, same goes for video game mags. On the other hand gun magazines tend to be...well, garbage, outside of the pictures and the rare article, which tend to inspire lots of in-store reading, which is frowned upon in a "get in get out) store like Walden.
Thirdly, there is no "policy" for shelving magazines. The only way to do it is tedious and time consuming and the person who ends up doing it tends to throw out whatever is on the top of the box in order to fill the racks so they can finish and go to sit around and complain for the rest of their shift. The way magazines come to the store is profoundly retarded and inefficiant; each store has room for 100-450 magazines depending on size, and the magazine clearing house generally sends out 300-1000 different magazines (generally with about 75 of the filthiest theft-inducing porn titles...if you don;t believe me, do a search on the interweb and you will find that there is now *digital* pornography for *FREE*!) which are selected fairly randomly by the person who is hated enough by the manager to do the worst job of the day (and because magazines trickle in throughout the week, you generally have a never ending job).
In the end, don't worry about gun rags not being at the store, not every hobby is going to be represented. Just go to a real bookstore and I am sure you will find lots of terrible gun magazines to read for twenty minutes.
