In Canada (which I believe is very similar to the US in this regard) lawyers generally can't contract out of their own negligence and the advice they give has to be at the standard of a competent lawyer (in the specific area of law). It doesn't have to be right or perfect but it does have to be the kind of advice you'd expect from someone who understands that area of the law.

Although of course, this post isn't legal advice. The actual rules are important, far more detailed than what I wrote, and jurisdiction-specific.

The reason why lawyers write "this isn't legal advice" (like I did above) is because it usually isn't and they don't want people to think it is. Although if it is, they could be liable for it and they want to reduce their effective liability by reducing the chance that someone will treat their legal advice as legal advice. Because sometimes it's not about what the law is but about what people think it is and how they react to what you've done.

Google "professional negligence" for more information.

I suspect the "this is not legal advice" disclaimer has more to do with the unauthorized practice of law. Depending on the state, it may be a crime, although I don't know that I've ever heard of anybody being convicted of it.