I also appreciate the fact that they want to eliminate their environmental impact but I think we can all also easily read the subtext: Printing only the exact number of copies required is cheaper and allows them – or their printer at the very least – to better plan their orders of raw materials. Financial planning with a fixed income stream is also much easier. In the end, all of this will help to delay the arrival of subscription cost increases which almost always results in a loss of subscribers and therefore a loss of income.
I did not want to say it, because I'm Mister Negative, but since no one else is...
In my experience, when a magazine goes 'subscriber only', it means it is in it's death spiral already. Not being on the news stand does save paper, I suppose, but it also means a lack of fresh eyeballs seeing it on the rack and making an impulse buy. This stops new subscribers from coming in.
Most of the time, such magazines plan (or believe or hope) that they can replace subscribers that drop off with new subscribers through referrals by current subscribers, like the kind of recommendations given out here on RFF. And that is true to an extent, but it is generally not enough.
Examples I can think of are "Boing-Boing," which is now web-only after going to subscriptions only, "The World & I," which is the same, and "No Depression," which is going straight from news stands to web-only without the intervening 'subscribers only' printing.
I wish Brooks Jensen and Lenswork well. But I doubt that 'subscriber only' will work, and I am guessing that Lenswork won't be around for another year. Of course I could be wrong, but my guess is based on observations of similar magazines in the past.