This would seem to prove that there are no non-suicidal rational thinkers living in France

How rational do you think this sort of remark makes you look? (Downvoted.)

What prompted that bizarre legislation?

It's hard to ever answer this kind of question precisely, but I suspect the case of the Martinots had something to do with the timing of it, triggering new legislation specifically to avoid a recurrence.

The more general force behind it is the pervasive meme that "our lives are not in our own hands", which justifies a number of stances: on abortion, on euthanasia, on suicide and assisted suicide, and on cryonics. The meme is, I suspect, more likely to occur as a tribal belief of people who identify as "conservatives" or "the right", and that's who has been in power here for a while.