Nobody should be drinking those to lose weight, because weight lifting to the point of needing extra protein is very much a way to gain mass, and doing both at once(gain mass and slim up) is extremely hard. Weight lifting being the most obvious way someone might actually need extra protein.

Other exercise, outside of extreme situations, doesn't need extra-high protein intake, so it's just excessive. Especially for lower intensity exercise that someone looking to slim up should be doing more of.

Also with how much extra junk a lot of those protein powders/bars/etc come with, it's just ill advised. Sugar, cholesterol, carbs, etc. Some are really bad choices in general vs just eating a normal meal.