If we are just talking about transcribing a new users posts in MathJax, I would say this can be a good idea.
There are different approaches on how to get somebody to adopt something. The idea to show a user by way of example how they should use the site seems in principle reasonable to me. Maybe even the not telling them what is not that bad. Every moderately attentive person ought to get curious what happened and maybe they'll be more motivated in a state of "Wow, how did my post get to look so good?" then in a state of "Argh. Read this, read that. On hold?!"
It is possible that some miscreants take advantage of the kindness of others in this way. But on the one hand some are more optimistic and positive and maybe do not want to think in this way, and on the other hand, if a user does this on a regular basis others will pickup on it, and if not sometimes they will be out of luck and run into users that hava a different approach to the subject and will close and down-vote the post, which is also a valid approach (especially if the user is not brand new).
I think we need to accept some variance in the approaches. Some users just do not want to operate with actions they see negatively such as downvotes etc. If the choice is that somebody does nothing about a bad post, or they objectively improve the post, I tend to prefer the latter. (I might make an exception to this if it seems to be done mostly to spite other users or to undercut their approach to moderating, but that's not that frequent I think.)