sender/receiver.setTransport(): Cannot replace closed transport(s)?

Section 5.3 (sender.setTransport) and Section 6.3 (receiver.setTransport) both contain the following sentence:

"If stop() has been called, or transport.state or rtcpTransport.state is closed, throw an InvalidStateError exception."

Why is it not possible for setTransport to replace an RTP or RTCP transport in the "closed" state? 
