But A is a common feature of almost all self-help camps - googling "Christian retreat testimonial" can be very enlightening (add the phrase 'changed my life' to the query for best results). I think most rationalists would be very skeptical of most of the camps that manage to get such glowing reviews from their participants. So P(B|A) - the probability that data shows a real long-term effect given that everyone loved it and is wildly enthusiastic - may not be much higher than P(B).

I suspect that even Christian retreats do cause B. Just being around people with high expectations of you will cause you to rise to the challenge. This is important since simply measuring B won't tell us whether the camp succeeded.