It's a pointlessly imported, Americanised tradition.
Plus it's a **** song.
Also I'm an anti-royalist.
But for them? Well...
If people from Liverpool feel marginalised by decades of London-led privatisation, losing jobs and, of course, that Hillsborough was never respected for what it is and the inquiry it deserved never came about, then I'd say that booing at the anthem is a fair and proportionate protest in response.