Rules GAFA creates do not apply to themselves.
Twitter and Facebook don't deliberately host illegal activity, and they stamp it out as best they can. Given that their have hundreds of millions of users in multiple languages posting billions of posts every day, it's a very difficult job and a lot of it requires users to report posts containing illegal activity so they can act on it. (And on that note, I have reported posts on Facebook for illegal or harassing content, and they took down those posts and acted on the offending accounts within 2 hours every time.)
I won't defend FB's or Twitter's algorithms. Based on recent reporting, it appears that internally employees are attempting to change the algorithm so hopefully something will come of that.
Parler and Gab have chosen to market themselves to groups known to engage in violent and illegal activity. Notably, moderate and liberal posts and accounts are immediately deleted or suspended on Parler, while right-leaning posts of any kind and degree of violence remain on those sites indefinitely. This indicates that Parler and Gab have the ability to moderate posts on the basis of content, and simply choose not to.