Children are not confined, they are free to move, to get in and out of bed. I have found my children are much more active when using a floor bed and slither around then crawl around much more, as their whole room is baby proof there is a greater area they are free to move about in. This is of benefit especially in modern times when children are often confined in a crib, to a high chair or rocker then to a baby car seat or stroller, yes some of these are necessary, however children need every opportunity possible for freedom of movement.
The most widely discussed Montessori sleep arrangement is the floor bed, a crib- or toddler-sized mattress either placed directly on the ground or upon a low bed frame. The idea behind this is that children can independently access (or exit) their beds at any time, requiring that the bedroom be fully child-safe.
The montessori floor bed, Promote independence. Children can get up and read a book, or get dressed, they do not need a parent's permission or help to do so. Over time and especially with an older toddler they can decide themselves when they are tired and lie down. For daytime naps my children would often put themselves to bed and go lie down when they needed to rest.