Shouting fire in a theatre is not covered under your right to free speech, as it impinges on the rights of others' safety and security.
It does no such thing. Now, panicking and trampling others in your haste to escape the "fire" does infringe others' rights, but that isn't directly caused by shouting "fire" and would not be justified even if the fire were real. The responsibility for that harm lies squarely with those who panic.
If someone fraudulently claims that there's a fire, and you take justifiable action based on a reasonable belief that they're telling you the truth, then they would be responsible for any harm that results from their fraud. That harm resulted from their choice, not yours. That isn't the case here; panicking and trampling others are not justifiable responses to a fire.
I find it hard to believe that so many people are willing to defend this bizarre ruling, which besides being unjust in its own right was also clearly politically motivated with the specific purpose of circumventing the First Amendment and permitting the suppression of political speech opposed to the draft (Schenck v. United States).