[trotsky]

Descriptive terms immediately describe the goods, or some important characteristic of the goods. Trademark law does not protect descriptive terms unless achieve "secondary meaning" in the minds of consumers. That is, trademark rights accrue when the public comes to associate the descriptive term with a particular company rather than the product in general.

[...]

Failure to "police" a mark by stopping infringing uses can result in the loss of protection.

https://en.wikipedia.org/wiki/United_States_trademark_law#De...

Two sponsors on the website, yet numerous parties issuing press releases:

https://encrypted.google.com/search?hl=en&q=world%20back...

[ignostic]

We don't know how many emails or C&Ds they've sent.

[hamburglar]

You don't have to stop use to stop infringing use. If you just want to protect your trademark, explicitly license it to them for free or a trivial amount.