Sure it doesn't stop someone wanting porn, but if you are trying to be good, but are in a moment of weakness, having extra roadblocks in your way that you have to jump over make you re-evaluate the situation and hopefully gives you more options to get out.
I think the filters are not for the person wanting to sin, but to help the person trying to be good so that he can be good. It sounds like this is the son being talked about in the OP