No one is impeding your free speech on forums; they're exercising their right to create a forum they desire and have no obligation to meet your bar (or mine) for what's too strict. But there's nothing stopping you from starting a forum and having no rules.
But even free speech, as protected by law, has restrictions. The First Amendment doesn't allow you to yell fire in a crowded building, spread secret troop movements or throw down what the Supreme Court has recognized as "fighting words," for example. Those can actions can be prosecuted criminally in the U.S., for instance. And there are plenty of things you say or post that can land you in civil court as well.
In some countries, laws are even stricter. When talking about Web limits, it's important to recognize that the whole world uses the Web, and what you're free to post in one country might land you in prison, or maybe worse, in another.