I get where you're coming from but it seems that your argument is different from what is going on in the NFL.
A directive from the boss about the content and quantity of my work is different from me disobeying a "should" statement about company policy in the company rulebook. If the boss told me I "shouldn't" wear the clothes I was currently wearing, I would investigate to see if there was cause for my boss' directive, and ignore him/her at my own peril.
This part of the argument is beside the point, however. Some folks are offended by the conduct of the kneelers, so they are using everything they can to prove that this behavior is somehow illegal, against company policy, etc. because they are offended.
The context of the policy is pretty clear. It is an expectation, not a suggestion. Just like a company policy that says that employees should leave all weapons at home. Not a suggestion.
People are offended because it IS offensive for anyone that understands basic human decency. When time is set aside to pay tribute to people to have sacrificed for others, it is offensive to hijack that time to raise "awareness" for whatever your agenda may be.