That's pretty accurate. If you really want to know, many of the carts are placed in pairs. There is a very thin metal divider tab that, on its own, might not hold a single cart from flopping around. With the second cart in place, it's secure. I don't think this applies to the metal carriers or single place carts.

There is actually a placard that states "all or no carts must be installed". That instantly makes it an FAA rule; although I believe it applies as described above.