Well, it's part of Atlassian's so called 1% pledge. The 10$ of the 10 user licences goes 100% to charity. It's donated to Rooms to Read. In software licensing this is called "freemium" model and very common to charge a small amount for the smallest user tier licenses.
To learn more about Room to Read and Atlassian's involvement, here are some links regarding the subject:
There is also a video where our Martin Seibert interviews Melissa Beaumont Lee from the Atlassian Foundation: