Solution to How to send a secure message in a box?

A wants to send a secure message via the postal service

A wants to send B a secure message. He puts it in a locked box and sends it via the postal service. The postal service is reliable and will get the box to its destination without the lock being opened and the message read. However, B does not have the key to open the box. How will he get the message?

Solution: When B gets the box, he will lock the box with another lock and send it back to A. A will remove his lock and send it back to B, who can then use his own key for the lock he used to open it.

Why does this not work if instead of a lock, the message is encrypted?