i agree with the others that this was just laziness. it was probably peppered with a little one-upmanship as well as i'm sure the cna got a kick out of showing a student something. i worked as a nursing assistant in a nursing home while i was in nursing school
years ago and, believe me, there are plenty of "shortcuts" that i've seen. the lotion should have been applied directly to the patient's skin with some massage. that serves two purposes. it stimulates the patient's circulation in that area and it creates a bit of a protective barrier on the skin for any future incontinence.
you have to realize that you are dealing with workers who are not necessarily learning the "why's" behind what they do. lpn and rn training is different. the way to handle a situation like this is to get as blank a look on your face as possible (in other words, try not to look shocked when you see these kinds of things!) and ask what that accomplishes in as friendly a tone as possible. if they can even give an answer i would then say something like, "i think it's much better to just take the time to do a little massage so i get all the areas back here covered," and smile. or, you can always do the "pass the buck" thing: "we have to do it this way or we'll get in trouble with our instructor." that way, you're being a good role model. when you get a license and you're in charge you can be a bit more assertive and give more specific direction if you are supervising this person.
when i worked in a nursing home i used to try to periodically take time to work side by side with the cnas making rounds. this is one of the best ways to find out just what kind of care they give, as well as what kind of bad habits they might have.