Because they need to insert themselves into the most private moments in order to keep control. A young person already feels embarrassed about changes in their body---it's just sort of a part of growing up---and now the new urges are most likely unexpected with their strength. What better way to gain control than to give them the impression that they are watched and should tell on themselves? Hey, if you can convince people to confess to such a private thing---telling on others will be no problem. It's not about masturbation--it's about control. It's about instilling a constant awareness of the GB in the minds of people. If they squirm during certain parts, then their brains will constantly be thinking about the guilt and the need to lean on the WT for a clean conscience. The WT is the only place to find a clean conscience. You need the WT to be good with God.
The account in the bible had nothing to do with masturbation! It was an issue about brother-in-law marriage---The guy didn't want to get his dead brother's wife pregnant cuz he didn't want to share the inheritence. The bible says absolutely nothing about this. Considering how common it is, you think it would at least mention it. It does not. Oh, so go on principal, right? Well maybe you can argue that point when talking about smoking---cuz it didn't exist in bible times. But frankly if you believe in creation, then be sure of this: Masturbation was created 3 seconds after genitals were created. If you don't believe in creation then this really isn't an issue.
I hate them for the shame they instill in young ones for natural behavior. So much unnecessary stress. I hate them for the shame they put on older ones too.
I actually had a distraught sister "confess" this to me! She saw me as mature for some reason. Well my heart just broke because she was so shamed. I simply told her that my opinion of her had not changed at all and I didn't think she did anything awful I told her it is not even mentioned in the bible and even the society doesn't consider it a disfellowshipping offense. And then I told her I really felt it was a conscience matter. I told her I didn't want her to feel bad anymore. It wasn't necessary. I again assured her that my view of her had not changed and she was a good person.
With my own daughter, I never went over the society's material on masturbation. I found it (the literature not masturbation) disgusting and guilt inducing and I didn't want her carrying that around with her. I was always too liberal for this religion and I'm not sure how I stayed as long as I did.
NC