Part of the decision seems to come from an association called I-Test which managed to lobby the government. The funny bit in this story, is that the association is composed by industrial who produces ethylotest!
If you speak French, you can have a loot to the source
Saoulez-vous citoyens ! | Causeur
As far as I have seen in France, the I did not know how much I drank is no excuse and won't get you anywhere with the police. This is only what I've heard from my friends and family who leaves there so may not be representative (most of them do not drink and drive

)
Goryl