(such as screaming fire in a movie theater when there is no fire)

This idiom comes from an analogy in a SCOTUS opinion arguing that checks notes it’s a violation of the Espionage Act to distribute flyers that oppose the draft. That case was later partly overturned in Brandenburg v Ohio and the standard is that speech isn’t incitement unless it is directed to inciting or producing imminent lawless action and is likely to incite or produce such action. To the point that “$SLUR should hang from trees” is probably protected speech (because the lawless action isn’t imminent), but “you guys, grab that $SLUR over there so we can string them up!” probably isn’t.

So defending free speech inevitably means defending white supremacists and the like because free speech doesn’t actually protect anything if it doesn’t protect upsetting, outrageous, or offensive speech (and likewise, the arbiter of what counts as offensive is not guaranteed to always be on your side). It’s why the ACLU has defended them on more than one occasion. H.L. Mencken put it best.

“The trouble with fighting for human freedom is that one spends most of one’s time defending scoundrels. For it is against scoundrels that oppressive laws are first aimed, and oppression must be stopped at the beginning if it is to be stopped at all.” ― H.L. Mencken