Riding the canter correctly and well in a full seat is difficult, and many more riders do it badly than do it well. As Allison stated above, it requires a degree of abdominal fitness, as well as correct position, relaxation and a good understanding of gait mechanics and how the horse's back moves. That's out of reach for a lot of recreational riders. I would much rather see an elementary or intermediate rider cantering in half seat, allowing the horse to move freely, than someone attempting and failing a full following seat and punishing the horse's back in the process.
There is nothing inherently insecure about riding the canter in half-seat or two point as long as the rider is in balance.