In a recent Tor blog post the following was mentioned:
The help desk has been asked how to configure a VPN to prevent a website from learning that a user is using Tor. We consider positioning a VPN between one’s exit node and the destination site to be totally unsafe, and not much more anonymous than using a VPN without Tor.
Now, having read the FAQ, I understand the reasons why it is preferred to not hide the Exit nodes, but none of those reasons seem to have anything to do with privacy or anonymity. How does adding a VPN after an exit node break Tor's model so badly that it becomes not much more anonymous than using a VPN without Tor?