It's wrong. Not because the stated probability at the point you stop the experiment is wrong, but because the (stupid) rule is that we'll approve the drug if the probability that the drug is better than the alternative is above some arbitrary threshold value. If you run experiments a fixed number of trials, you'll get a variety of conclusions with different strengths. If you stop as soon as you are in the "barely passing" zone, you'll get a lower number of failing result, a higher number of barely passing results, and none at all that do better than barely passing.