There is one big difference between France and the US.
As far as I know, France has never given the WTS official status as a religion.
The US has; so would have to rescind that status first.
Note this recent case:
WATCHTOWER BIBLE & TRACT SOCIETY OF NEW YORK, INC., et al. v. VILLAGE OF STRATTON et al.
certiorari to the united states court of appeals for the sixth circuit
No. 00-1737. Argued February 26, 2002--Decided June 17, 2002
http://caselaw.lp.findlaw.com/scripts/getcase.pl?court=US&vol=000&invol=00-1737
The basis of allowing JWs to go from door to door was the principle of freedom of religion found in the First Amendment. The US Supreme Court indicates that they had ruled for the WTS for the last 50 years as a group covered by that amendment.
Freedom of religion seems to be approached differently in the US than in France as well. There are many people and groups in the US that while they don't agree with the WTS religious ideas, will fight for the concept of protecting freedom of religion for other groups.
I can't see the US government going after the WTS without going after other religious groups.