If you are granted access to the compiled version to a piece of software covered by the GPL then the GPL grants you the right to request source for that software. For Wix to do what you suggest would also mean they would have to price the compiled app out of range its intended audience would pay thus making the app pointless.
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Don't get me wrong, but doesn't the gpl allow for (or at least not stop) you to charge for the code?
https://www.gnu.org/philosophy/selling.en.html
Couldn't wix just charge some ungodly sum and be done until someone ponies up the $$$?
If you are granted access to the compiled version to a piece of software covered by the GPL then the GPL grants you the right to request source for that software. For Wix to do what you suggest would also mean they would have to price the compiled app out of range its intended audience would pay thus making the app pointless.