Read the letter published by Ars[1], they refer not only to the logo but to the "Ubuntu word" (trademark of Canonical) and "to use the Ubuntu trademarks and Ubuntu word in a domain name would require approval from Canonical".
It was very polite, and props to them for that. But that doesn't mean there were no threats.
to use the Ubuntu trademarks and Ubuntu word in a domain name would require approval from Canonical... Unfortunately, in this instance we cannot give you permission to use Ubuntu trademarks...
This has a big "OR ELSE" implicit in it. Requiring means enforcing.
[1] http://arstechnica.com/information-technology/2013/11/canoni...
Edit: formatting