If, as you say, the main audience for papers written by SIAI is through SIAI's website and not through the journals themselves, why spend the time and expense and hassle to write them up in journal form? Why not just publish them directly on the site, in (probably) a much more readable format?

I imagine that some academics would be more willing to read a paper from the website if it is also published in a journal, especially if they are writing their own journal articles in which they would prefer to cite a journal article to a paper published on a website.