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.
“The single raindrop never feels responsible for the flood.” - Douglas Adams