The fact that so many people care this much over whether a guy sits or stands is a bit sad to me. He has his reasons, whether or not you agree with them, and he has a right to express how he feels under the US Constitution (within reason, obviously, and he's well within his reason). You could consider this as a peaceful protest - he didn't disrupt anything, he didn't yell "FIRE!" in a crowded building... He kept his ass on a bench while some music played.
No, the team has no right to let him go based on this, unless there's something in his contract regarding standing during the anthem.