Apparently, John Piper thinks it's idolatry:
https://www.morethancake.org/archives/7643
To be honest I think calling it idolatry is just silly.
There's also something of a distinction here. I can see the point that paying someone else to write something and then presenting it as if you wrote it yourself is lying. In that regard I wonder why the ghost writer would be willing to write an entire book and then give someone else all the credit for it. What seems more likely is that a ghost writer might write a blog post and not get due credit for it, in which case it might be argued that it's immoral to leave the implication open that the blog owner wrote it but the whole thing comes down to a matter of opinion.
Buying intellectual work is a very broad topic so it's not necessarily helpful to get hugely bogged down in this opinion of a very specific scenario. The example I think of is the software I've been writing lately. At present it says "copyright (my name)" on it, but if the company I've been working with decides they want to buy the intellectual property in it (and offers a sum that makes me willing to sell) then my name comes off and the software says "copyright (company)". All that means is that they bought the copyright to it - if they then offer it to their customers the fact it lists their name on the copyright statement doesn't say that they wrote it, just that they own the copyright to it.
Likewise if I take a photograph I have the right to put "copyright tango" on it and restrict use. If you decide you want to own my picture you can buy all the rights to it from me, at which point you can put "copyright lucian hodoboc" on it. Putting your copyright notice on it doesn't mean you took it, merely that you own th rights to it.
In the context of what John Piper wrote in the link you posted it looks like he's more concerned with the idea of people writing something to give a false impression, in which case we might look at the motive behind doing something. I can see the reasoning that it's immoral to pay an expert in a field to write something under my name so that I can look like an expert even if I know nothing about that field but frankly the person who tries that is going to get caught out sooner or later anyway.
If someone is sufficiently busy that they choose to pay someone else to write all or part of a piece of work for them it's entirely possible that they don't have the time or feel the need to credit every little part of it. In a scientific paper it is expected that research that is quoted is referenced in detail; in an academic publication it is expected that references will be quoted in a bibliography. In something like a Christian book it's appropriate to credit another published book that is specifically quoted but if an unpublished work is quoted then the difference between "someone else wrote this chapter for me" and "this chapter is based on a discussion with the guys at church" immediately becomes vague. If I wrote a book that had large sections based on discussions with people at church I wouldn't expect to do much more than give them a mention in the "acknowledgements" page at the front.
