Well, I hadn't been aware of these rules at the DU and they're nicely Stalinist, or maybe it would be better to say Maoist.
There is one, though, that I've seen in other places, even ones of opposite political/ideological leaning (ie, like us but not us). This business about not being able to post that someone is lying. This is pure absurdity. If you know someone is lying, then you post it, just as you'd say it. And just like in real life, you'd sound like a whiny kid if you couldn't back it up, and people would ignore you.
But simply outright banning naming someone a liar is crazy. Sometimes people are lying and need to be identified as liars. Sometimes they're lying about 99.8% of the time, like the 981 members of the DU.
The word and the label are hard, and you should be wise enough not to use hard words without provenance, but sometimes the truth is hard. It's good enough for the law, isn't it? The truth is an absolute defense against a defamation suit. That is, it's not defamatory to say someone is a liar when they are a liar.
It's not like naming someone a liar is some unprovable and highly uncivil vulgarity. It can't, for example, be established if someone's a censoredmothercensored, that will always be a matter of opinion and private posting boards can set their own rules about subjective opinions in vulgar format. It can be established if someone is lying or not, and if someone is lying, he is a liar.