Where is the empirical evidence from engine/airframe manufacturers to warrant anything but zero tolerance?
Two points spring to mind.
Firstly 'zero tolerance', as you put it, would presumably mean no flights at all in any areas with any quantity of ash in the air. Taking this to the logical extreme, a single particle of ash in the entire European airspace would be enough to ground everyone. That's 'zero tolerance'.
Secondly, would this approach also apply to other know risk factors, such as bird strikes and thunderstorms, such that all flights in any areas affected by birds and CB's would also be banned? 'Zero tolerance' again?
Flying involves some risk (everything does). Establishing an acceptable level of risk is what the airline industry does every day, and what most people do themselves every day. It's 'risky' to drive to work, but most of us accept the risk because we live too far away to walk. Would we drive after no sleep for 36 hours, or when drunk? Most of us would say no, because the risk is much higher, but we can't demand a 'zero tolerance' of risk on the roads or none of us would ever get in our cars.