I'm afraid that's how it normally should work with teaching. Schools pre-exist and you have to go work at the ones that require teachers. That's why in the UK it's the young, inexperienced teachers that are put the through the gauntlet.Indeed it was voluntary...
BUT
A lot of urban schools were selected because of the teacher / pupil ratios. A lot of these were white schools and these teacher's choices were voluntary retrenchment or re-deployment as decided by the department.
Take it or leave it.
So voluntary yes...
A more sensible approach would have been to offer them the payout for moving to a rural area for some minimum contract period. At the very least there should have been no payout for quitting. Don't like the deployment options, then fine, quit teaching, but you don't get any money for doing so.
Problem is that the government appears to have thought there were more than sufficient teachers, plus plenty being produced by training colleges, and that the existing teachers were more dedicated to their job than they actually were. And what they probably did is pay those teachers moving costs to go overseas. Of course they would have been in a for surprise if they went to the UK, because foreign teachers also get shoved into the worst schools until they've proven themselves and can get a better school to take them on.
But the bad move was made, so now work has to be done to fix things.