Traditionally many journals require or accept data supporting the science. This is physically separate from the main text and published on a web site as {Supporting/Additional/Supplementary} {Information/Files}. For toll-access (closed, paywalled) publishers this information is often outside the paywall and publicly visible and downloadable.
Many publishers informally accept that this data belongs to the commons and is not covered by copyright.It is physically possible to download and aggregate this and republish as an added-value resource [1]
However other publishers (e.g. J. Neuroscience) refuse to accept supplemental information. Yet others require it to be sent to repositories. Some of these are open (e.g. Dryad) while others (e.g. crystal structures in Wiley/Springer/Elsevier) are deposited in the subscription based Cambridge Crystallographic data Centre.
[1] 250,000 supplemental data sets from ca 150,000 articles have been aggregated in Crystaleye by robotic means and given added value through search tools and domain-specific display: http://journals.iucr.org/j/issues/2012/02/00/he5533/ describes
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