Disrespectful by definition.  US Code (as well as common sense) states proper protocol to show respect when present at the playing of the national anthem is to stand.  That's the standard, the tradition, the right thing to do.  Therefore to intentionally and purposely REFUSE to stand - particularly when you're front and center on the stage for the event - is clearly a sign of the opposite of respect (i.e. disrespect).  

It's intentional disrespect with the goal of creating controversy and the extra attention is brings.