It Is Your Responsibility To Act Against Infringement Of Your Trademark
The United States Patent and Trademark Office (USPTO) grants registered trademark status but is not responsible for halting the infringement of registered U.S. trademarks. This is your responsibility and even applies in the infringement of common law trademarks, as well.
If you intend to use your trademark in countries outside of the United States it would be a good idea to file in those countries using a single application made possible by the Madrid Protocol. This will prevent others from filing for your trademark in foreign markets.