I can think of thousands of semi-commercial journals that have dealt with this issue. All are academic (we're talking highly specialized, and have personal subscription rates running at around 100+ bucks), but the way they deal with things is a moving wall of around three years. What this means is that they only put content that is older than three years online, for subscribers to access freely. Given the nature of the academic environment, they are (I am reliably informed) of the opinion that pdfs will be passed around, but they are fine with this as within certain limits it constitutes fair use under copyright law and they are aware that there is a very limited market for back issues.
The AHA could adopt a similar approach, and have a way for members to access old issues while not making the more recent ones available for non-subscribers to rip off (but are AHA members going to put Zymurgy out on bittorrent sites, given that most of us aren't just in it for Zymurgy?). While I appreciate that the AHA would like to recoup costs through selling back issues, it is prohibitively expensive to buy many issues.
I also feel I should add that the 'zymurgy archive' section on the website leads one to believe that you can get old issues there, but this is not the case...There are loads of articles I'd like to read that I never saw, and recipes I'd like to get, but all I can see is when they were printed!