The situations are morally different because politicians’ job is politics and governance; it doesn’t emphasize word mastery, it’s more purpose-, goal-orientated.
On the other hand, artists and craftsmen demonstrate their skill at their art or craft, for which they are praised in varied degrees and on which being an artist, artisan or craftsman is predicated. It’s dishonest to misrepresent one’s skill level in such circumstances, and thus it is necessarily dishonest to voluntarily assist another in doing that. This is similar to cheating at school or in other examinations, e.g. bar exam, medical exam, state exams, etc.
Undisclosed professional assistance to a fledgling writer on technical matters or advice on how to develop the plot, flesh out ideas, how to structure the narration, how to do this or that better, in a more engaging manner, that’s all fine. But delivering a ready work just so another can sign his name on it, nope, that’s dishonest.
I face the same problem at work. I’m a translator. I know some of my clients are middlemen, so it’s assumed they’re going to subcontract and outsource, and the goal is to cover the workload, ensure better service by involving a more specialized colleague, etc. etc., or even to build the renown of their small translation company, which is still fine. But I can’t know when they’re contracting me for work they’re intending to pass off as their own to appear more personally skilled at personal performance of the actual work (vs simply getting it done) than they really are. And that’s where I have compunctions.