He has dual citizenship in the US and Germany, and he could have chose to play for either. By earning a cap with the US in a major competition, he is now tied to us for the rest of his international career.
There are other rules that go along with that in terms of the player's age and the ability of some to make a one-time switch. My understanding though is the Boyd is now locked in.
This also speaks to what's significant about a player like Tim Chandler (who could be very helpful at the back) turning down his invite to camp. He doesn't want to be locked in to the US, despite the fact that the Germans have no interest in him at this point.
I'm not sure how much interest the Germans had in Boyd for that matter, but unlike Chandler he has been unequivocal about his interest in playing for the US. He looks like a promising player that can add some danger to the US attack in the future, and hopefully in this cycle.