Given a journal policy that authors must share original data, am I free to use this data without granting original authors authorship?
I'm not a fan of "mandatory authorship" on published information: I feel that once something is published, one should collect one's rewards by means of citation rather than by strong-arming people into giving you an authorship.  Mandatory authorship on already-published data feels to me too much like a form of salami-slicing on a dataset.

That said, the journal's policy does not appear to say anything at all regarding authorship.  Therefore, the default position would seem to be that the authors must share the data with you (as mandated by the journal), but are also free to require authorship as "payment" for sharing.

The journal, however, may feel that this goes against the spirit of their data sharing agreement, and if so, then you may be able to obtain the data without being coerced into authorship.  I would thus recommend, like @BrianBorchers notes in the comment, that you write to the chief editor(s) and ask for a ruling.