Re: abusive sensei
Thin ice here because like Mary said "abuse" is a loose definition and is further complicated by persons who may also have an emotional response to the topic...
First, I think of abuse as containing a key component of deliberate misuse of either or both authority and action. Second, victims of abuse are typically under a power of authority or action that curbs their response behavior. Not always, but you see these factors in most abuse scenarios.
In martial arts training, I acknowledge "abuse" when you have the deliberate misuse of training on behalf of the abuser and a limited response from the abused. The scenario described in the thread appears severe but may be a better illustration of poor instruction and/or a poor dojo environment than abuse. While [it seems] odd, students within the dojo may prefer severe training and accept the risks associated with that training. The fact that you do not prefer that type of training indicates that dojo is a poor choice for you.
Negligence is a serious issue and I think we should keep in perspective when there is "abuse".