He did have one.  Like everyone else, he made the mistake in believing the Republicans, after seven years, would have one ready to sign on January 20.  They are the ones who lied and misled people on this matter.
They are disrespecting the flag, just as much as burning the flag does.  Both are Constitutionally protected speech, but their actions are disrespectful.  Just because they have the right to do this does not mean that they should.  And it does not protect the players from criticism or reaction. 
People will not pay $500 to take their families to a kids game played by grown men and be insulted.
They are in their working environment, not on their own time.  Let's see how their vision of "social justice" pays their bills.