It's signed by the US trade representative, not Obama. I don't know if this kind of thing is approved all the way at the top, but the letter uses "I" a lot suggesting it was his decision. I think the headline should at least be changed to "The Obama Administration". Also, it's not a "veto".
The law puts it in the hands of the President, see this subthead: https://news.ycombinator.com/item?id=6153056 ; he may of course delegate his authority, but the buck stops on the desk in the Oval Office.
