Ultimately, I understand it as this.

Especially in terms of psychological effecting drugs

If the people getting the placebo get better (in the case of psychological drugs.. feel better) as often as those on the actual drug, then it's quite likely the drug is doing nothing and both sets of patients are getting better because the belief of taking a drug that makes them better is causing them to get better.

(apologizes for the confusing paragraph)

And hence, the drug is deemed worthless and thrown out, time to get on with a trial of another drug.