I assume only the regular version (non-Pro) is open source? If not, I'm really curious how this works together with Pro version being paid.
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.Well, they can, they just have to ship the source to the buyers (and no, they don't need to give it to non-buyers).
But the odds of someone except the original developer wanting to work on a paid version are low anyway.