Asking because I don't know - I see this a lot, but is it necessary? Surely we don't live in a time where a commenter on a news site is liable for their opinion, right?
As far as I can tell, this is the result of people reading lawyers announcing that they are lawyers and posting such a disclaimer, and other posters specifying "I am not a lawyer" purely out of consideration for the reader, not out of legal obligation, and getting the two all mixed up together, while deciding that both are necessary (I'm quite sure one is not, and I'm pretty sure the other one isn't exactly necessary, either).
Consider how many published pieces and TV news programs feature people who are and are not lawyers, providing opinions about the law, typically with none of these disclaimers. It's evidently not a problem. I think lawyers only bother to do it in forums because there's some remote chance that someone might be able to argue they were mislead by the conversational and two-sided nature of the medium into thinking the lawyer had taken them as a client or was offering advice in an official capacity (I'm skeptical such a case would get very far, in any event, though).
(Also it is fun to add “for external use only” etc disclaimers.)
I cannot imagine getting pulled into court because I recommended someone file in small claims court for $PROBLEM, and didn’t put “IANAL”.