Because, it may not be a real disease but Ritalin is a real drug. If you take a mind altering drug you can expect your mind to be altered. You would see an effect if you took cocaine or LSD or whiskey too.
In this case, Ritalin is prescribed for people with certain behaviors and the drug does have the effect of reducing those behaviors. The issue is, are the behaviors due to a medical condition or are we just drugging people who act in ways we preferred they didn't? I think it is the latter. There is no medical condition that causes ADHD, there are energetic children who lack discipline (among other causes). Drugging them is wrong.