Anakin may have been playing by the rule of association. If you know that somebody is suffering, and you know it's individuals from your tribe, your family, your friends, etc. who are causing that suffering, many people will argue that you are as guilty as the actual tormentors for your negligence of action. It's sort of the idea that simply by lacking ignorance toward a certain wrong doing, you become an accomplice by silence alone. What is to say all the Tuskens saw it happen though? Plus, women and children? Even if they did know, it isn't like they could stop Tusken Warriors.