It likely is a result of "copy-catting" to some extent. For those that _are_ lawyers, giving legal advice without first clearly stating to the recipient that no implicit attorney-client relationship is being created can create just such a relationship (i.e., and therefore potentially subject the atty. to possible malpractice charges, etc.). So actual attorneys always preface any statements with the "not advice" comment. Because of that, some non-lawyers may have picked up upon the practice and started restating it as "I'm not a lawyer ..." from some belief that there is a legal requirement to do so.