Emphasise that there is no right or wrong method
I don't think it is worth giving a prescriptive set of rules on what to do and what to avoid. Learning is a very personal process and what works for one person may be useless to another. I think the most useful thing that a teacher can do is to describe as many different techniques or variations as possible, and let the student decide what works for them. It may be that highlighting the whole text just helps them to read by providing something for the hands to do. I believe some dyslexic students find the bright colours more helpful than a black and white page.
Encourage your students to examine their own learning process.
Encourage them to think about what it is they are trying to achieve via highlighting - help reading in the short term? Long-term memory retention? Make reviewing the article at a later date easier? But hopefully, if the purpose is to pick out key points, then by making them stop and consider this consciously, they will be able to draw their own conclusion that highlighting an entire page is not helpful.