Because Sarkozy is somehow less anti-American (Anglosphere?) than the rest, or because they've gotten tired of paying for their own
All of my life I've been seeing the anti-Americanism of the French, the sneering poseing of someone who's national language sounds like someone straining on the toilet.
The Socialists and Comunards who rewrote their WWII history and ignored the 60th anniversary of D-day.
So now they decide that after America elected a Socialist, it's OK to be a NATO member and all the American money that'll come their way- for doing nothing but signing a piece of paper.
Personally, I'd rather have a former Soviet satellite as a member- one who *knows* what liberty is about.