I don't know if this is actually been a problem ever brought to a judge to be honest, but that's the reason why that disclaimer is made and of course, I am not your lawyer.

Everyone is of course free to speak about legal matters and provide their take, but keep in mind that even well studied lawyers, with all of the context and evidence are not always successful in their arguments. Internet forums are many steps removed from these professionals, so you should moderate accordingly.

In forums such as Reddit/Hacker News/etc, there are many discussions that concern legal matters - in my experience the overwhelming majority of highly voted comments and frequently repeated ideas are not ones which are grounded in fact, but rather comments that align with the groups desires. People upvote and repeat ideas that appeal to them, not to ones which they've investigated themselves as factual or accurate.

Another one to look out for is IANAL: "I am not a lawyer". (Yes I'm being serious with this acronym.) This is to avoid a different kind of liability: the unauthorised practice of law.