Some journals upload their accepted papers for public viewing shortly after the papers were accepted, examples of this include J. Math. Phys and Cell Systems. What's the reason for doing this if the journal needs revenue?
I don't know about either of the two journals you specifically mention, but in math things like this are sometimes done to attract more high quality submissions to the journal to raise it's profile, particularly for new journals, since academics generally like open-access, and it can be a selling point when choosing a journal. (From what I remember, with new journals what I sometimes see is that the journal is open-access for its first couple years.)
I would suspect that they do that to attract some attention to the new articles. Once the attention has been established, they put them behind the pay wall.
They may have other ways of generating income. Also Open Access 'advertises' their publications so why shouldn't they be placed out there for everyone to read and learn from.