You are absolutely right, an opt-in system would destroy the whole location-by-wifi-ssid service for everyone.

...but that doesn't create leverage or a good reason for having such a poor and inelegant opt-out. This is an "ends justifies the means" argument, which is in itself Machiavellian.

From a different perspective, there are other ways Google could handle an opt-out list - such as letting people add their SSID/Mac Address to a central opt-out registry they maintain.

This would be trivial to implement, but I would suggest that Google have intentionally created a high-friction solution of "-nomap" to your SSID simply to make it as difficult and unpalatable as possible. And that really stinks.