Millions stand to honor the flag and country it represents.  Refusing to stand is not showing respect.  That is precisely why they are not standing.  Ask them.
To see professional athletes publicly dishonor the flag for which hundreds of thousands of Americans have died, the flag that millions of Americans have seen drape the coffin of their child, their spouse, their sibling, their parent, or other loved one is as morally repulsive as it un-American. Of course, these players have the right of free speech — and so do I, and that is precisely the right I am exercising now.