I agree, but don't think JSTOR in particular can do a lot about it. Rewritten to be more accurate, what "entrusted to us by the owners and creators of that content" means is "licensed to us by the copyright holders". In most cases the copyright holder is a journal publisher, and they control what terms JSTOR is allowed to make their files available under. The only major exception are public domain works, and they have been moving in the right direction on that: http://about.jstor.org/service/early-journal-content
I do think JSTOR could institutionally be run in a more progressive manner. They can't fix the problem, but they could put a bit more pressure in the right direction. I'm hopeful that moves like Early Journal Content are showing some signs of that, though I could just be overly optimistic. But in either case, ultimately they aren't the ones who can make the decision to do anything about the post-1923 content. For past content, the journals who hold the copyrights are going to have to be convinced to open it up, and for future content, academics are going to have to start publishing in open-access venues in the first place.
I do think JSTOR could institutionally be run in a more progressive manner. They can't fix the problem, but they could put a bit more pressure in the right direction. I'm hopeful that moves like Early Journal Content are showing some signs of that, though I could just be overly optimistic. But in either case, ultimately they aren't the ones who can make the decision to do anything about the post-1923 content. For past content, the journals who hold the copyrights are going to have to be convinced to open it up, and for future content, academics are going to have to start publishing in open-access venues in the first place.