Simply, if you enable sideloading/downloading from non App Store, you make it very visible in the UX that you are essentially in a "non-safe mode" (the actual security status probably varies, but Apple would probably go out of its way to hype up the potential of danger here), and you consent to all sorts of disclaimers disavowing Apple's responsibility here, and recommending they make backups before doing so. In fact maybe they could distinguish between pre- and post-sideloading enablement backups.
If users complain after sideloading was enabled, the Geniuses can simply tap on the sign.
