I don't know for a fact, but I doubt that this specific detail is enshrined in law. Much more likely is that the law requires certification from a bureaucrat that you are not carrying some terrible disease.

Greek bureaucrats have a lot of leeway and very little accountability, and their positions are openly coveted (among a portion of the population) for ability to extract bribes (it's very desirable to be a tax assessor, for example). These requirements may just be the bureaucrat's opening offer in the bribery negotiation, which can then be made "unnecessary for an upstanding citizen such as yourself".