In a similar way, removing read permissions on a file won't prevent reading of the contents through an already open file handle. That may seem insecure, but the property can be used to enhance security. If a process needs to read from or write to a protected resource, it can start life as root, open the resource appropriately and then drop privileges. The now unprivileged process still has privileged access through the open file handle.
"Even if you are on the right track, you'll get run over if you just sit there." - Will Rogers