Spencer's constitutional right to free speech was not, and in fact could not possibly have been, restricted, threatened, or infringed upon in any way by being punched for what he said.
I'm not sure off the top of my head about the legal precedent here, (and apologies if this was brought up earlier in this thread) but I feel like it COULD be a first amendment issue if anyone in the justice system, from police officers through to judges treated this attack differently from other battery cases due to the nature of Richard Spencer's speech and beliefs. It wouldn't be exactly the punch by itself violating his constitutional right, but the punch and its treatment by the state together.
All that said, yes, it pains me to see equivocation between the legal right to free speech and the ethical value or "Human right" to free speech.
The former is clearly based on some concept earlier of the latter. If you live in a country where a police officer will arrest you for a speech act or an angry mob will beat or kill you for it, both have the same effect. I suppose in the latter you hypothetically have the recourse of the law, but that has limited utility after the fact.
I personally value that unpopular ideas can be voiced. I think most of us agree that the government should be restricted from deciding which ideas can be shared, and I'd like to hope that we can agree that angry mobs shouldn't get to decide that either, even if some of the mobs happen to agree with me or you about what speech is terrible.
I like my violence relegated to self defense, rule of law and consensual BDSM. Anywhere else and we start to look pretty dystopic.