fkv
We map what we see, and when there's no sign regarding legal access restrictions, we don't map legal access restrictions. It's as simple as that.

Your assumptions are obviously rooted in the http://wiki.openstreetmap.org/wiki/Barriers page, which you referred to in the forum thread. That page is not a map feature page, and the line with the access=no default was inserted by a single user without prior discussion. It's just nonsense.