There is a law in France (and in most west europe countries as UK, Germany etc) that prohibit racist/sexist/religious/etc discriminations and hate encouragement.

Unlike USA, most european countries put boundaries to free speech. It do not mean that there is censorship. Nor it mean that it's illegal to think that jews stinks or whatsoever. You can think whatever you want, and tell whatever you want in your private circle, nobody will arrest you for that. But if you use your _public_ free speech to propagate these ideas you will have to face a court.

I know it can seems strange to US people, just like the ability to buy an AR-15 in a WallMart seems strange to us...

But that's our law, and it have been democratically voted.

Now the interesting question in the article is: Do French people that tweet are subjet to French or US laws ? Should Twitter obey to the laws of it's users or only to american laws ?

"All the French are racists", "French should filter web content at ISP level", etc subjects are just bullshit.