A law professor who directs an adjunct-taught legal writing program e-mailed me this, in response to my article on the subject:
[T]he phenomenon you describe is one of my biggest hurdles. I spend a significant amount of time warning my adjuncts not to "correct" anything they can't demonstrably show, through a reliable source, to be wrong. I also advise them to do just what you say: mark sticky usage or spelling as unwise, perhaps, or likely to inflame passions but not "wrong" unless it truly is, and to look everything up before marking it even if they're certain they are correct. I still see "forgo" "corrected" to "forego" when it means to go without, and phrases like "have carefully reviewed" edited to put "carefully" after "reviewed," for who knows what reason. Sigh. Even if these were actual mistakes, our students certainly have more important things to worry about in trying to improve their writing....
[Also,] I occasionally am hired by big law firms to tutor associates in legal writing. One downtown firm asked me to put on lunchtime presentations for the associates, but to begin with a session for the partners, to train them how to be effective writing coaches. I thought this was a wonderful idea. Among the things I included in that presentation was a general admonition along the lines of what you wrote: don't mark personal preferences but rather stick to actual mistakes, for all the reasons you cite as well as some of my own. A partner objected strenuously. He said, and I kid you not, "One of the reasons I became a partner was to impose my personal preferences on others." I was not asked back.