because although most ISPs will be honest, some won't be.
Where have you been for the past 10 years? Most ISPs (read: Telcos & Cablecos) have long demonstrated their inability to be honest.
And while transparency is certainly important, its only the first step. What the most NN people want is transparency + nondiscrimination based on traffic source. If you have no or few alternatives for internet access, it does very little good knowing that your ISP is screwing you.