There are certain operations that only "kernel" packages are allowed to do. If that weren't the case, then this would indeed be little more than an obscure quirk.
Ah I see. So non-kernel packages do not have access to native modules at all? I believe in Elm native modules were still possible, just not publishing them to the package database. But in this case I guess you wouldn't be able to put a native module in a GitHub repository and then pull this in as a dependency, which is equally rubbish.
Elm doesn’t allow kernel code in externel packages. Gren is performing the exact same check, it’s just that Gren uses Git as a package manager and so has to do it in a different way.
Edit: EdwardDiego's answer clarified this for me