The magazines available on this site are all professional magazines. This also means that the audience for these magazines are highly skilled and niche. It also means that the market for these magazines and more importantly, the target for the audience is specific. The publishers, in order to better justify the resources invested in these publication will prefer that the publications are used by people who are qualified professionals in their own fields. This way, the advertisers in these publications also get a very targetted market.
In order to achieve this, these magazines, white papers, ebooks, webinars etc are made available free of cost.
So the net effect is a win-win-win situation for all three parties concerned. The publisher gets quality subscribers, the subscriber gets high quality information for free and the advertisers get a highly optimised target audience to market their products and services.