a) The First Amendment of the US Constitution, which guarantees free speech, explicitly does so in a political context ("Congress shall make no law [...] abridging the freedom of speech"). Imzy is not the US Congress, they don't have jurisdiction over anything but their own little niche of the Internet (which absolutely nobody is forcing you to visit if you don't want to) - so why are you upset when they ask you, within the bounds of their community, to be nice?
b) Why does free speech mean that we get to insult and demean each other, even going so far as to sending death threats? I think the German constitution (the Grundgesetz) has a point when it says in Article 5, §2 that the right of free speech is limited, amongst others, by every person's right of dignity ("Recht der persönlichen Ehre" - a hard to translate concept).
c) On Reddit and like sites we see behaviour that we would never tolerate if we saw it in real life. Why do we not only tolerate it online, but suddenly elevate such antisocial behaviour of the extremest kind to a mystical status of "expression of free speech"? There is a reason we are taught "manners" as children: without a minimum of politeness, societies cannot function. Politeness and courtesy have to do with respecting another's person even if you don't share his (or her) opinions, with tolerating people who are different from you. And is that not the very epitome of freedom?
Free speech is perhaps the most fundamental right of any democracy that must be protected at all costs. It is vital for the continuation of a free nation. However, Imzy is not a nation. It is a little website on the Internet. And it doesn't ban dissenting opinion (the real reason why we need free speech) - it merely asks you to remain polite while stating your's. So what is your problem with it?