So, as an older Duke guy who has been around a long time... your take on this is a bit "hindsight is 20/20."
K took a lot of crap in the years after taking the US head coaching job. He took it in 2006. In 06 Duke went to the Sweet 16. In 07 we went 8-8 in the ACC and went out in the round of 64. In 08 we went out in the round of 32. In 09 we went 11-5 in the ACC and out in the sweet 16 again. And in that time our top recruits were Gerald Henderson, Kyle Singler, Elliot Williams, and Ryan Kelly, while missing on guys like Durant, Wright, Patterson, Monroe, Wall, and a host of others. There were a NUMBER of people, both at Duke and nationally, who said that taking the USA job was a mistake, that it was stretching K out and Duke was suffering for it.
In terms of recruiting yes, it EVENTUALLY helped... but it helped because he was successful, and when a coach is successful, why WOULDN'T it help his recruiting? It feels a bit putting-the-cart-before-the-donkey by saying otherwise.
And K isn't the only guy who benefited from taking a national job. Cal took the Puerto Rico job to land KAT. It worked out very well for him. There have been a number of coaches who have taken national gigs where they actually coach the guys they are recruiting. I don't see people getting up in arms over that. My guess is that K's success is why.
As for why K took the job... I get that some folks might always see things in a negative way. But K is a veteran of our armed forces who has always been a vocal and active supporter of the men and women in our armed forces. Duke has done many programs with our soldiers, almost every year in fact, and K has maintained a very close relationship with a number of the people he served with and knows, such as former Chairman of the Joint Chiefs of Staff Marty Dempsey, General Vincent Brooks, and General Robert Brown. Many of the US team players have spoken about how K really developed the idea that they were representing their country and, in a way, the men and women who defend our country, and the pride that developed with that. You can think the guy is profane and prickly and has a huge ego and can be a prick (and I would agree with all of that) but I think his love of country is genuine.