Their policy is not only to simply follow the law: they are also actively paying (with advances) writers with a specific agenda denying trans people rights. This is not about what you think this is.
Even if they were, it’s my impression that the majority of the popular Substack contributors are on the opposite side of the issue. So the current outrage seems more like people being mad that those with different viewpoints are allowed to speak.
Interesting phrasing. They would probably define their 'agenda' as:
"kids can't decide to get sex changes"
or
"people who are in the midst of, or recovering from, gender reassignment are perhaps not the best choice for soldiers"
or
"hey whatever but I'm not paying for it"
And aren't the haters you and Tankus portray them to be. Substack may disagree and ban them, as is the trend in big tech, but their positions are way more in line with US (and world) opinion than Tankus and company.