I think it may stem from laws and rules related to practicing law, but there is certainly not any "requirement" to stipulate you are not a lawyer when speaking about legal things. However there are a few things to be mindful of: In the (all of the) United States (and many other countries) the profession of lawyer or attorney is regulated and requires a license to practice as such. In many places to practice law without a license is a criminal offense. Obviously this doesn't contemplate criminal charges against a group of people spouting off about something legal related on the Internet (otherwise we would have to jail perhaps 80% of reddit), but providing legal advice and interpreting the legal effect of contracts, statutes, or regulations is part of the practice of law.

The reason we heavily regulate the practice of law is similar to the reason we heavily regulate the practice of medicine. If you screw something up, the consequences can be bad. If someone takes your advice thinking you know what you are doing, and you are wrong, that person can end up a lot poorer or even in prison. (Though some will argue we regulate it so lawyers can be the only ones charging high fees for legal advice!) Just like one probably shouldn't go around telling folks not to worry about their incredible chest pain and shooting arm pain unless they are a trained medical doctor, the same could be said for those that go around offering home spun legal advice.. ("Yeah man, if you ask the undercover cop if he is law enforcement, he HAS to tell you! Otherwise it is entrapment man...")

Also the reason you always see lawyers preface everything by saying "I'm not your lawyer.. This isn't legal advice... Yadda yaddda yadda" is because once there is an Attorney-Client relationship a lot of stuff happens. The lawyer has a host of duties to the client, from confidentiality, to competence, and many others. That obligation isn't taken lightly and many times if there is any doubt whatsoever whether or not there is an attorney-client relationship, the law will find in favor of one because --- well -- the lawyer should know better. Therefore lawyers will make it annoying clear they aren't your lawyer before spouting off some deep legal thoughts about whatever the topic of discussion is.

It may have started as an ass-covering move, but I also think of it as a common courtesy for letting people know that you're offering non-specialist advice and to take it with a grain of salt.

I kind of wish people did that for more things: "I am not theologian/windows user/empathetic person, but..."

Great answer. Has the acronym "IANAL" ever been tested in court in such a way, to your knowledge? Now you have me curious, and that'd honestly be quite amusing.

Ha! Interesting question. I just ran an all state/federal Westlaw search for "IANAL" and didn't hit any case law. Two or three briefs, a couple of scholarly articles, but no court opinions. I also searched for "I am not a lawyer", but that brought up about 130 caselaw hits which seemed to primarily be references to various filings by pro se parties actually saying in a court filing "I am not a lawyer".

That lines up with what I assumed, and the last paragraph is specifically what I was asking about (not the IANAL case so much as the opposite). Thanks!