If your platform is essentially a giant loudspeaker, and you see the public believing too many lies, and the lies causing damage, then it's reasonable to at least think about not giving your loudspeaker to people spreading lies.
(I also think it's generally bad, but that's my attempt at a steelman.)
I think it is basically this. Amplifying dangerous messages has a liability.
Screaming "FIRE! RUN FOR YOUR LIVES!" in a crowded theater when there is no fire or evidence of fire is not protected speech under any sane "freedom of speech" doctrine.
Giving the person screaming that a megaphone makes you complicit in the crime.
Giving people the "freedom" to trick other people into causing harm to each other is not giving people freedom. It is supporting abuse.
There are lines, of course... fine or otherwise, and it is a messy process to draw them. And mistakes are often made and should be called out.
But it is not a weak argument to claim that supporting the spread of disinformation about public health measures during a pandemic is supporting abuse.
And the problem with social media is that instead of there being 200-500 people who can hear the megaphone there are hundreds of million. Scale matters. What can be tolerated in the local pub a half hour before closing may not be tolerable on a billboard in Times Square.
(I also think it's generally bad, but that's my attempt at a steelman.)