What's online and what isn't?
The answer usually comes down to copyright. Items published in the United States prior to 1923 are usually out of copyright. In the United Kingdom, copyright for most works lasts for the life of the author plus 70 years.
Items that are out of copyright become part of the public domain, and can legally be scanned and posted online.
Wellesley purchases access to additional collections of texts, most of which are not available for free online.