This is an excellent question which applies to a lot of machine learning datasets. AFAIK there is no specific licensing of much of it (licenses you see are generally attached to the labels: the images/text/etc are often not even part of the download and you need to go scrape them yourself) and it's often claimed that the results of the network are free from the copyright of the training data used to create it, but this is contentious and has definitely not been tested in court.