You are both correct. It's a federal law and it doesn't say it is advisory in its actual words, but that is because it doesn't have to say it's advisory. It excludes enforcement provisions and is not penal in nature because it cannot be legally enforced due to the First Amendment's free speech guarantee. Thus, a person does not break the law by not complying with it. It's considered a 'custom' law, the codification of what they deem proper custom that people ought to follow but are not required to do so. However, the government and other employers can require their employees to follow it when performing duties pursuant to the employer/employee relationship.