Finally I see someone realizing the "hovering" seats in a train. Anytime I sit in a train, I wonder why they have not done so for the last 30 years or so. While they state its for luggage and design purposes, I just think: Its so much easier to clean. Even automatically ;)
My guess: Ease of installation and for stability reasons. Bolting the seats to the outer frame probably requires much more consideration than simply bolting them to the floor.