I think the obvious (and thus probably too simplistic) answer is that this is the same France that had enacted an absolute prohibition against overt displays of religious symbols by schoolchildren.  While the UK and Germany are generally as secular, neither one has the same depth of feeling regarding walls between church and state that has been a recurrent feature of French society since the Revolution.  My hunch is that this goes a long way to explaining why they're the most sensitive to this perception of the U.S.

What's really notable, though, is that if one looks at the dataset, the French actually perceive America as "too religious" to a lesser degree than they did in 2003.  While we're not talking about a huge shift, the figure in 2003 was 65% viewing the U.S. as too religious (compared with "only" 61% in 2005).  Again, this may have been influenced by their own experience with the recent student prohibitions.