But even disregarding that, I don't get why you are baffled: The GPL is a hack intended to spread freedoms. Many who support the GPL do not support it out of some desire to respect IP laws, but as a means of reducing the use of more restrictive licenses. E.g. to maximise access to knowledge or maximise the ability to modify and use data.
Wanting the data in JSTOR freed up, possibly regardless of copyright status, is entirely consistent with supporting the GPL in those cases.
For some this is a moral or ethical issue - it is perfectly possible to consider the current state of copyright an immoral restriction of personal freedoms.