Read the GitHub EULA. They do not currently claim ownership of uploaded content. However, they claim the right to modify the agreement: " GitHub reserves the right at any time and from time to time to modify or discontinue, temporarily or permanently, the Service (or any part thereof) with or without notice. ... GitHub shall not be liable to you or to any third party for any modification, price change, suspension or discontinuance of the Service.
A classic example of a formerly open database becoming closed is CDDB, the track list database for audio CDs. Once open source, the current owner, GraceNote, doesn't even allow clients other than their own, and inserts ads. Another example is Google's takeover of the historical netnews database and closing it to full downloads.
To avoid that, it's important that creators limit the rights that some service gets merely by hosting the content. A service like GitHub could claim that as soon as someone else makes a check-in, the aggregated content becomes theirs. The GPL, which covers derived works, prevents that.