In light of some of the recent issues in the environmental community:
It may be good to have a list of datasets that the government has/uses that can't be released and the reasons why.
Ie. we can't release dataset x because it contains dataset y which we purchased from company z (and due to contractual/legal reasons we can't release dataset y).
Another example would be data that is provided from other countries with the condition that it be not released.
A third example would be datasets that are not available due to national security or other political reasons.
In these cases clearly spelling out where the data is coming from becomes useful for people who want to go create that dataset for themselves (either by purchasing the dataset, or by trying to establish contact with the country of origin).