It's not true that TGV trains have always accepted bikes. Some TGV trains do, some don't. There was a wide geographic variation regarding bikes the last time I rode a TGV with my bike, which was 5 years ago. For whatever reasons, TGV trains between Paris and the southwest of France generally did not allow bikes in boxes, but TGV trains to many other regions were more likely to officially allow them. I've got a folding bike so I was able to bring mine on 2 trains between Paris and the SW. However, in the wake of the Madrid train bombing, the SNCF (French railroad company) was requiring all passengers to bring their luggage to their seat area. The large luggage racks at the end of the cars had been blocked. The upshot was that large items were dumped in the platform areas outside of the seating areas, as well as blocking the aisles inside the seating area. Everyone, conductors included, seemed to go along with this. I don't know if the luggage areas are still blocked off.

What you described wasn't so much being "allowed" to bring a boxed bike, but rather "getting away" with bringing a boxed bike.