I believe reddit comments are made under a CC license
But it doesn't specifically address collection and distribution of API results as a dataset.
(From the page:)
A licensing agreement is required in order to:
* use the reddit API for commercial purposes. Use of the API is considered "commercial" if you are earning money from it, including via in-app advertising or in-app purchases. Open source use is generally considered non-commercial.
* use the reddit alien logo ("snoo") in your app or for its thumbnail. Any new apps you create must be approved as well before usage. The circular "r" logo is reserved solely for use by reddit, Inc.
* allow users to subscribe to reddit gold via in-app purchases. If your platform allows for it, we encourage you to work with us to make this happen. We see gold purchases as a way for you to help reddit and to give back to the reddit community.
Do any of the apps actually do this, though? (Especially the free ones.)
"You retain the rights to your copyrighted content or information that you submit to reddit ('user content') except as described below."
And the exceptions just state that Reddit has a perpetual irrevocable worldwide license.
So it seems like there's no default license and others don't have any automatic rights to use the content. Does this assessment seem correct? In practice, it may not be a big problem, particularly for academic research and such, but I'm guessing there are some uses that might cause problems.