JSTOR does not have the power to direct what others do. If the publisher chooses to restrict a document, JSTOR has no say in that decision--JSTOR is merely a repository. JSTOR has access to essentially every academic article in existence precisely because it does not attempt to enforce any political goal on contributors.
If you want to change what publishers do with articles which should be in the public domain, get the law changed. But don't hold JSTOR responsible for something outside of their control.
The law already is what it needs to be. JSTOR has limited access— and continues to do so through their terms of services— access to even old documents which are already lawfully in the public domain.
They have operating costs. They're a non-profit but they pay people to digitize the works. It costs a lot of money to access some of these things. It's not prohibitively expensive to access these documents. You can read three articles free every two weeks if you're exceedingly down on your luck.
You couldn't a week ago. This isn't some accident. Their openness comes hard won with threats of their reputation.
Their digitization is largely done by a for-profit company. It's not competitively bid. There are many parties who would digitize these works at no cost— archive.org and google for example, and could afford to do it without pay walling the results.
Anyone at all (including you) is free to scan journals that are in the public domain and publish them online for free. If you scan/ocr them then there are many places (including Project Gutenberg and archive.org) who will host them for you.
If you want to change what publishers do with articles which should be in the public domain, get the law changed. But don't hold JSTOR responsible for something outside of their control.