Ithinkso,
I agree with your sentiments about 0.0 alcohol, but when I asked about it a few years ago (in a different context) I was told that the tests can read upto 0.02 even if you have never consumed alcohol in your life, just through the respiration process, which is why 0.02 is chosen... it is effectively 0.0.
With the drug tests, I guess we'll have to wait and see how it all plays out.
I'm sure if the airlines keep getting screwed over with crew not being able to operate due to false positives the system will change quite quickly.