how can a rostopic has publisher 'none'?

edit

Hello, I am really new to ROS. I am trying to create a namespace around the topics publisher by a 'fetch' robot. But some topics like /cmd_vel are always staying outside the namespace. I tried rostopic info /cmd_vel and that gives publisher as 'None'. How can a topic be published if there are no publishers? There are subscribers to this topic as well and I have checked that there is some data in this /cmd_vel as well, the subscribed topics are responding to that. But why is the publisher shown as None?

Please help me with problem. Thanks in advance.