Almost everyone that I know that reads Singletrack mag (myself included) is a subscriber… It's loads cheaper that way, gets delivered to your door, and you don't have to think about going to buy it. Also the subscriber only issues with their clean front cover are much nicer to look at than the ones you can buy in the shop.
For that reason alone I think Singletrack mag have shot themselves in the foot (or maybe they haven't, but they've certainly shot their resellers in the foot), as why would you pay more for something when it's also harder to obtain?
As a bike shop, personally I'd take the executive decision not to sell any mags (unless on sale or return), though I think a "shop copy" for customers to browse through whilst in the shop would be a good touch. Certainly having a shop copy would likely keep them in the shop longer, and also maybe get their mind thinking about spending a bit of money at least…