A plausible path is precedent and normalization, not zsh specifically.

If a widely used copyleft component (like a shell) starts being accepted as “OK to lock down” in consumer or embedded devices, manufacturers and courts get comfortable with the idea that user-modifiable software is optional rather than a right tied to distribution. Over time, that erodes enforcement of anti-tivoization principles and weakens the practical force of copyleft licenses across the stack.

Once that norm shifts, vendors can apply the same logic to kernels, drivers, bootloaders, and userland as a whole—at which point locked-down embedded devices stop being the exception and become the default, even when the software is nominally open source.

It's called tivoization and started with a device called "Tivo" which was the first of its kind to attempt this procedure.

There are probably lots of hardware devices in your house that use GPL software but prevent you from actually modifying it because the hardware will refuse to run modified copies. If a piece of software is licensed GPLv3, it would violate the license terms to do something like this.