A requirement? [IANAL]
Not exactly but the advertiser is trying to avoid claims against mis-representation and fraud...and "legalise" their advertisements/commercials.
Essentially, these "non-patients" are Shills.
In marketing, shills are often employed to assume the air of satisfied customers and give testimonials to the merits of a given product. This type of shilling is illegal in some jurisdictions but almost impossible to detect.
It may be considered a form of unjust enrichment or unfair competition, as in California's Business & Professions Code § 17200, which prohibits any "unfair or fraudulent business act or practice and unfair, deceptive, untrue or misleading advertising."
By adding the disclaimer (in really small type) they essentially add a layer of absolution against fraud amd misrepresentation claims.
Advertising (in the US) is "governed" by the Federal Comminications Commission (the FCC)..
However, the courts ruled that freedom of advertising, which constitutes a part of freedom of speech, can be regulated more strictly than any other form of expression. Thus, state authorities are entitled to control the distribution of false or misleading advertising, advertising of illegal goods or services, and truthful advertising, if public interests are in question.
Advertising in the United States is regulated not only by state authorities, but also and mainly by means of self-regulation.
Other countries and jurisdictions will have their own systems and legislation.