I can't speak for other companies but we make salary offers based on the candidate's experience and role, not their location.
However, the job security is an interesting point. I suspect that Zapier gains huge leverage over it's relocated employees because they're now dependent on their Zapier job, another hard-to-find remote gig elsewhere, or funding their own move back to a tech hub. I don't know what Zapier's pay bands are like but I'll bet their employee churn is lower than industry average.
Yes, and that is an unhealthy way to manage churn in most cases. Perhaps Zapier is different but I'm skeptical. I think it's more likely that a higher than typical fraction of their workers are dissatisfied but locked in for reasons you note.
As for wages, the concept of "cost of living adjustment" is just a way for owners to keep more of the profits at the expense of employees. If web/mobile app shop X in the Bay can pay $120k for a freshly minted BS grad, they can pay that for a remote dev living in Milwaukee or eastern Europe, and it's very likely a company based in Milwaukee could do the same. They don't because of the leverage, though.
For me I've yet to see a remote position that wasn't fraught with the same risk as any other company, or that pays anything like a reasonable salary.