France’s reaction to the Mauritania coup, I think, can be boiled down to the fact the rest of the world reacted so negatively. French intell. likely had a certain view (not necessarily favorable, but not resistant), where as for political reasons (domestically and internationally) the Élysée has adopted a very different one. So blatantly backing the overthrow of a democratically elected leader in a region where such men are few and far between is no longer politically correct, even for the French. That there is so little at stake in Mauritania didn’t help the case against Sidi either. If the French had come out in favor of the coup, the world would ask questions about why they were so adamant over a country of little geostrategic value (the country doesn’t even have a major Islamist threat to try and sway the rest with; yet). If the condemnations out of Washington, Addis Ababa, Brussels, Moscow, and just about everywhere else (except the Arab League) had not been so resoundingly negative, the French would have stuck to their traditional tenebrous complicity. It would have damaged the country’s prestige to take such an outlier status.
There are also domestic political matters to consider, such as the Socialist Party’s reaction against which Sarko is likely posturing, but this is a blog about Third World politics and geopolitics, not French internal squawking.