Just to clear things up a little here. Pianobook is a non-exclusive library. People can submit their sample packs to Pianobook whilst also hosting them on their own website (e.g. as Jon Meyer does) or other website/libraries. They can choose to give their library away for free on Pianobook whilst asking for money on their own website.
The terms that we came up with for Pianobook were simply that people, other than the original contributor, cannot redistribute the sample library obtained from Pianobook. This provides a level of control for the contributor around the distribution of their work. There is nothing to stop someone from approaching the original contributor to gain permission to redistribute. Indeed, this is exactly what happened when Spitfire Audio redistributed some of the libraries as Spitfire LABs instruments.
On the flip side, we do ask contributors to agree that people can use the libraries submitted to Pianobook in music productions including any commercial music. This encourages people to use the hard work of those who submit to Pianobook without concern for their own music distribution. We ask contributors to only upload contributions where they own the rights to distribute the recordings within their submission, again so we don’t end up with people redistributing a commercial library in some form on Pianobook.