Playing the national anthem at game openings is a choice made by the stadium and the franchise. The business could also say a prayer or recite a poem. No action should be dictated in response to any of these.
Many players come from all over the world. They're brought to the US to improve the teams people love. And when they are asked to pay hommage to your national anthem, they do so. Wheter they are standing or kneeling shouldn't be an issue, because they're already paying respect to a national anthem that's not their own.
Playing the national anthem isn't just a preference of the league, it is a custom that all professional sports recognize.
The business, stadium, or franchise is at liberty to take "action" that perhaps "should" be done. In other words, the action needn't be dictated by a third party. Furthermore, nobody is seems to be suggesting that other parties ought to intervene.
The NFL could make it a condition of employment to stand at attention when the host country's national anthem is played and flag is displayed. This is true in U.S.A., Canada, Mexico, U.K., etc.