Looks like the license is AGPL so that nobody can clone them and do a similar 'pro' version.

Now, when you contribute to their OS project you need to sign a CLA which pretty much consists of the usual 'all your base are belong to us'.

  You hereby grant to the Owner and to recipients of software distributed by the  Owner a perpetual, worldwide, non- exclusive, no-charge, royalty-free, irrevocable  copyright license to reproduce, prepare derivative works of, publicly display,  publicly perform, sublicense, and distribute Your Contributions and such derivative  works.