HathiTrust provides reading access to all materials in their collection when allowable by current copyright laws. You may have noticed that while there are over 17 million items in the collection, there are 6 million that are available full-text. The reason for that discrepancy is that some materials in the collection have been placed there for archival or for individual campus access purposes but may not be in the public domain.
In-copyright search results are marked as "Limited (search only)" and are not available in full text. Even though these works are not available full-text and are fully protected by copyright, you can search within the full text of the work to find out how many times a particular word or phrase appears. This type of search may help you decide whether or not you want to seek out a physical copy, but it also enables researchers to access the entire collection from a data perspective to conduct projects like text mining.
If the documentation provide by HathiTrust regarding their copyright and take-down policies is confusing, feel free to reach out to your subject liaison/archivist or our HathiTrust liaison for assistance.