Why do I see "Cannot Use Thunderbolt Accessory"? I purchased a brand new MBP and a cable was included in the box, it has USB-C type connectors on each end and works for charging the computer.

When, instead of charging with the brick, I plug it into my new iMac, I get this error message:



Why do I see this? How do I fix the problem?

This message implies that I have connected the "accessory" (iMac) using a non-Thunderbolt port on the MacBook Pro. However, the marketing materials on apple.com tell me that all the ports are Thunderbolt ports:
To slow charge your Mac Book Pro from your iMac, you can connect the USB-C Cable. Unfortunately It seems with recent updates is shows this error on both the MBP and iMac screen. However despite this happening, charging is still occurring.