I think you're confusing the issue; OS processes are preemptive, threads within a process are not. Preemptive threads are not the norm on Unix / Linux / Windows

I think you're confusing the issue; OS processes are preemptive, threads within a process are not. Preemptive threads are not the norm on Unix / Linux / Windows systems. The BEAM VM implements its own threads which allows for them to be preemptively scheduled.