According to PLAIN's website itself, PLAIN is "A communication is in plain language if its wording, structure, and design are so clear that the intended audience can easily find what they need, understand what they find, and use that information."
It is not to ensure clarity for citizens consumption specifically, but to make it easy for the audience to understand and use the information, whatever that information is or is aimed to. There's nothing there about adhering to letters of the law nor overseeing productions. In the end, PLAIN language is meant to make everything clearer and easier. That includes laws and contracts.
a. to make legal and governmental documents easier to understand