Horse hockey.
Kalmbach publishing is large and diverse enough that they're not running razor-thin margins on the specialty magazines - otherwise they'd fold them just like Mainline Modeller and Model Railroading were folded by their publishers. In fact, one likely reason those magazines folded was that their advertising to content ratio was lower than MR. Having watched the Model Railroader content:advertising ratio shrink from greater than 1 to less than 1 over the last 30 years, I find it hard to believe they're not making money - especially considering what their advertising rates are.
Second, we're talking about magazine articles that are already set, edited and photographed being ported or scanned to a PDF file. In fact, looking at the list of PDFs, most of them are material that have been re-hashed through several articles and books - in other words they've been long-since paid for. Given that the publishing industry has been electronic for years, we're talking about the work of a few minutes for an intern, not a major project for the editorial staff.
Does it cost money to maintain their slick website? Yes. But I suspect they're large enough that they do their own hosting, and have their own support staff, so the overhead, while large, when spread across all the magazines published by Kalmbach is relatively low.
I'm not suggesting (unlike some people in other forums) that the material should be free. I'm suggesting that it's grossly over-priced for what is being offered.