We live in a society where there is a potential for a tremendous amount of backlash for a variety of reasons if you intervene.
Safely and efficiently protecting that victim, without placing yourself in undue harm, is illegal. Because you would have to strike first before they knew what hit them. That is a risk of a murder charge. Or manslaughter. Or other various charges for violence and injury. Especially if you mistake the situation and the couple were just role playing. You never know. Do you have a job, a home, a family? Protecting what you have against the risk that society condemns you for acting weighs heavily on us all.
Take the controversy of a
police officer saving a girl from a knife in the heart. He had ZERO time to react. Did a heck of a job stopping a murder as the fatal blow was being delivered. He is vilified and at least a small contingent want him crucified. Do you want to be in that position, where doing the right thing puts a target on your back? On your family's back? As an employee we are trained to look the other way. To not intervene. Call and wait for others. That is our society at large. Even those who are supposed to protect others are instead attacked for doing so.
Passivity is beaten into us all, and to an extent it certainly works.