From JSTOR's September 7 press release:
[T]oday, we are making journal content on JSTOR published prior to 1923 in the United States and prior to 1870 elsewhere, freely available to the public for reading and downloading. This includes nearly 500,000 articles from more than 200 journals, representing approximately 6% of the total content on JSTOR.
And their rationales from their FAQ:
Our mission involves expanding access to scholarly content as broadly as possible, in ways that are sustainable and consistent with the interests of our publishers who own the rights to the content. We believe that making Early Journal Content freely available is another step in this process of providing access to knowledge to more people; that we are in a position both to continue preserving this content and making it available to the general public; and this is a set of content for which we are able to make this decision.Posted by stemp003 at September 9, 2011 11:22 AM
Copyright term outside the United States is set at the life of the author plus 70 years. We believe that 1870 is a reasonable date to assume that all copyright is expired.
We do not believe that just because something is in the public domain, it can always be provided for free. There are costs associated with selection, digitization, access provision, preservation, and a wide variety of services that are necessary for content to reach those who need it. We have determined that we can sustain free access and meet our preservation obligations for this particular set of content for individuals as part of our overall activities undertaken in pursuit of our mission.