I'm disappointed this wasn't a 9-0 decision. I'm also disappointed it had to go to the supreme court. These protesters are clearly assholes, but freedom of speech is absolute.
Legally, freedom of speech is not absolute. Alito invoked the fighting words doctrine, which has been upheld (though in narrowing interpretation) by the Supreme Court on numerous occasions. I'm not saying it would quite apply in this case, or that I disagree with your sentiment, but there are limits established by precedent to the freedoms defined in the Constitution. Whether we agree they are valid is another matter.
It seems clearly wrong that a funeral should be interrupted by anyone for any reason. Don't families have the right to bury their loved ones in privacy? The court should not defend the rights of one group if by doing so it strips another of theirs.
Does this apply to whistleblowing? A whistleblower certainly strips others of some of their rights, eh?
The families have a right to a certain level of privacy, but this took place in public and protests were given restrictions on how close they could be by local authorities.
It's absolute in the sense that you absolutely have the right to express an opinion at any time.
I'm aware of the whole "fire in a crowded theatre" concept and as you said, that doesn't really apply here.