The person who believes (or says they do) that the earth is flat, has every right to do so. I agree with you, they're objectively wrong. I find it arrogant to say they have no right to speak because of that, and will not.
> protect videos promoting and glorifying beheadings
To play devil's advocate, "objectively" these beheadings are still happening somewhere in the world. It's an undeniable truth even though you don't want to see the evidence for it. For a lot of people this is their flat earth. No evidence means it's not happening.
Why not allow the content to be seen, let the world be horrified that there are people driven to become like this, and try to understand what led them to do those things?
ISIS and other violent groups based outside the US were indeed the catalyst for the first big wave of content removal from American platforms, as I recall it. I noticed it particularly during the Syrian Civil War. For the first few years of the war you could view combat footage on YouTube and Twitter without difficulty. Later you started to see such videos rapidly removed, particularly if they were filmed from the POV of anti-government (often some Islamist faction) forces.
At the time, conservative media outlets seemed predominantly critical that American platforms weren't censoring enough content. Even if that content wasn't actually illegal to publish under US law.
Here's a 2014 article from the conservative site WorldNetDaily, for example:
"ISIS-branded merchandise sold on Amazon, Facebook"
Dave Gaubatz, author of “Muslim Mafia: Inside the Secret Underworld That’s Conspiring to Islamize America,” contends Facebook and other social media sites have a responsibility to shut down the forums used to market radical Islam and mobilize Islamic youth.
“I am a defender of the U.S. Constitution but the Constitution was not designed for enemies of America,” said Gaubatz, a former U.S. Air Force investigator.
“We must stop jihad online because these groups target our children through social media,” he told Fox News.
...
“ISIS is more than likely not producing shirts but indirectly they are benefiting from it,” Scotty Neil, a former Green Beret who founded Operator, a clothing company geared toward special ops soldiers, told Fox News. “I don’t think that T-shirt company X is sending the Islamic State funds, but people wearing these shirts are making an outward statement and that often starts a dialogue and debate that furthers their message.”
Advocating to join a terrorist organization, or providing support for such, is illegal. Believing or saying stupid things is generally not illegal. Equating the two is nonsensical both from an intuitive and legal stance.