Github != open source. There are plenty of proprietary software products like HighCharts that distribute with GitHub and have a clear commercial license (https://github.com/highcharts/highcharts/) -- merely saying GitHub is not enough to communicate the license terms.
We communicate the license terms by using the license (and the website, and the FAQ, and the Terms of Service, etc), not the link to GitHub. :)
I just realized the GitHub link goes away on mobile (for lack of horizontal space) but, there are many other places on the website where it mentions that the project is open source.
