Does this all rely on the honesty and integrity of the person that "has symptoms" to only raise the alert "when they have symptoms" and not "when drunk" or "it'll be a laugh"?
Since the impact of raising an alert will be for others, known and unknown, to take protective isolation actions then the mis-use of alerts could have serious consequences. Not least if/when they finally are infected and try to tell their boss, it could be the "that's the third grandmother's funeral this year" situation.
Even if I turn off location services, or opt-out sharing my location, contacts, history... who really knows whether those choices are ignored. It's like the office thermostat that everyone tweaks, but which is unconnected to the HVAC control system. Can we actually believe Apple/Google/Microsoft/Facebook/etc., etc?
This is akin to those knee-jerk "think of the children" campaigns to rid the world of unassailable encryption. Humbug.