please enlighten us to how a zero tolerance of ash which has been in place for years can suddenly be thrown on the ash heap within days and replaced with new levels of acceptable ash.
you are reading way too much into the words.
In order to continue you need to define how you intrepreted zero tolerance.
From a safety of flight standpoint take a look at the FAA release posted here a couple of days back.
Here's a typical question from an operator to an OEM
1) Is it OK to fly into volcanic ash ? ......
ans: No
2) What do I do if I accidently fly into ash ?
ans: inspect your aircraft and engine systems and clean and replace as necessary
3) Why is it not OK to fly into ash?
Ans: because it may damage your engines or aircraft by plating out in the hot section of engines, blocking small holes with ash or eroding aircraft windscreens, and/or blades in an engine.
4) Is it unsafe to fly into ash?
ans: it might be if etc. etc.
5) How do I know if I have flown into ash?
ans: any of the following symptoms may occur etc. etc.
6) How much ash does it take to bring down an aircraft?
ans: Don't know, it has never happened when the following steps have been taken. etc. etc.
7) How do I avoid flying into volcanic ash?
ans: ask a meteoroligist
No where in the above is there a statement of zero tolerance and aviation has not operated in a zero tolerance regime but rather chosen the route of avoid with caution and accomodate if an inadvertent encounter occurs.