No, I've been to Restrictions and the icon is not hidden.
After some research, it looks like the issue comes from the fact that the tablet was offered to me by a friend who lives in Qatar. I am afraid Qatar is one of those countries where FaceTime has been blocked at government's request.
Even though I am French and I live in France, it seems that nothing can be done to remove that restriction. Very silly indeed, I don't understand why Apple cannot figure out the geographical location of the tablet owner based on his ISP and let him download the app if he is located in Europe. After all, nothing could prevent a Middle East national to buy the iPad in Europe with FaceTime activated, and then move back to his Home country.
Evidently, the protection based on serial number is not fool proof, the ISP detection would certainly be a better tool, but by choosing serial numbers Apple punishes many of its Western clients ! I was contemplating the purchase of a Mac, but this is upsetting me so much I'll stay with Windows ...