Talking to someone on a chat line and started thinking about this.
Some possible reasons Ranma allows the abuse to be done to him.
1. It's a slapstick comedy and Ranma is generally the one who is hit.
2. Ranma has been abused so much that he has come to expect abuse or at least gotten so used to it that he no longer really cares. I know that's a sad thing to say but its true he was hit and insulted by his father for most of his life in the name of training.
3. He appears to allow people to hit him when he feels guilty. This is particularly true with Akane and Ryoga.
4. Doesn't know how to retaliate. Ex. When Nabiki is manipulating him. Hitting her would be against his moral code because she isn't a martial artist and weak and he doesn't know what way to retaliate.
5. He's too forgiving and can't hold a real grudge.
6. His live in the moment way of thinking.
7. He has very bad luck.
Anyone dispute these or have anymore possible reasons.








