This was my question to. I suspect they're worried about someone making a paid version with extra features which aren't contributed back to the community.
For a moment, I was thinking Defold ought to dual-license their engine under both the current non-OSS modified Apache and the GPL. That way, you'd have the option to either:
1. Commercialise software created using modified versions of Defold, without releasing the source of your modified version, as long as you don't commercialize your modified version of Defold itself.
2. Commercialise a modified version of Defold, but you must make the source available under the GPL. (Which would mean that source could be used by the upstream project as well.)
But while typing this up, I noticed the flaw in this plan—the parenthetical isn't true! Because the upstream project would be dual licensed, they couldn't use GPL licensed code.
For a moment, I was thinking Defold ought to dual-license their engine under both the current non-OSS modified Apache and the GPL. That way, you'd have the option to either:
1. Commercialise software created using modified versions of Defold, without releasing the source of your modified version, as long as you don't commercialize your modified version of Defold itself.
2. Commercialise a modified version of Defold, but you must make the source available under the GPL. (Which would mean that source could be used by the upstream project as well.)
But while typing this up, I noticed the flaw in this plan—the parenthetical isn't true! Because the upstream project would be dual licensed, they couldn't use GPL licensed code.