whereas i agree that (from the pov of the first amendement) the governement should not impose restriction on free speech, IMHO that is not the issue here. The free speech right does not include a right to force other private person to listen to you. I see no reason not to have certain building as no-demonstration zone, independent of the speech (that is , not only restricted to those guy but to anybody to do a picketing) , and that would include cemetery. I dunno if it is the same in the US, but cemetery here around have a right of dignity respect of the beraved visiting thier dead, and thus you can't make a raucus or disturb the ebraved.
It seems essentially Scotus says free speech is so sacred, that the beraved has no right not to want to be disturbed by it.
Essentially I think that is one of the multiple reason I disagree with free speech, but agree to the right of expression.