Is it acceptable for a Political leader to shun the National Anthem?

[The Fury]

I don't think the issue here was to do with not signing the national anthem per se but the place he was at the time. A memorial for fallen soldiers. Regardless of your view on Monarchy, a political figure is there to pay respects, and many of these soldiers sung that anthem with pride. I mean, he didn't even bother doing up the top button on his shirt.