Meta is a private organization[1]. It is not “the US.” In fact, most of its customers aren’t even in the US.
Private organizations dictate their own social constructs. It’s not acceptable to proposition sex at a book club, but it’s acceptable to do that at a swingers club.
This idea that private companies follow the social norms in the US is only partially true at best.
It’s socially normal for me to talk about how my company sucks and I hate my boss with 100% of my friends and family, but doing so at my place of employment isn’t okay. That’s one major example where my company doesn’t follow social norms.
If what you say is true then YouTube should allow hardcore pornography since over 90% of men watch pornography. It’s socially and legally acceptable, with numerous legal brick and mortar businesses specializing in that market. But they don’t allow that because they are a business that doesn’t want to engage in that business activity.
I think it’s easy to mistake the current state of private social media as “open” in the way you describe just because it looks open. To your point, not a lot of speech is banned outright, which seems parallel to US social norms, but whether or not something is banned or removed from social media is almost meaningless because promotion and multiplicative reach is the domain of the algorithm.
The algorithm doesn’t promote and multiply the reach of content based on whether that content is most aligned with social norms. It’s promoted based on whether it makes money or causes users to stay watching the platform.
In real life, you don’t hang out with your friends and family most who make you the most mad or generate the most shocking content. But that’s how social media works because social media isn’t real life.
This idea that today’s social media platforms are a neutral vessel for speech is flawed right from the start because of that algorithm.
[1] publicly traded, private as in “private property”