Here's why:
* Metonymy is a figure of speech where a word or phrase is substituted for another with which it is closely associated.
* In this sentence, "The White House" is used to represent the administration or the people within the administration who make the announcement.
* "The White House" is not literally announcing the treaty signing, but rather, the people within the administration are.
Let me know if you'd like more examples of metonymy!