This is a valid criticism of every exercise: done improperly, anyone can get hurt... done properly, there is still a small chance of injury.
Exercise is important for everyone's health, but the fact remains that the human body is poorly cobbled together and still not well-understood by science. Do what you can to help yourself stay healthy, but there is never a zero risk of critical failure.
The difference is that when I go to my regular trainer, he is very specific about not letting me exceed my physical limits. He's also very conscientious about making sure I know my physical limits so I don't hurt myself when he's not around.
In a yoga class, the instructor can regularly instruct people to do things that are inherently dangerous (even if they aren't painful.)
Pushing the risk onto the students is not particularly helpful. The student has placed (or misplaced, depending) their trust in the instructor not to hurt them. It's the instructors job to make sure they don't get hurt, including not having them do inherently dangerous things.
You'd be surprised how much science understands about how the body works.
Exercise is important for everyone's health, but the fact remains that the human body is poorly cobbled together and still not well-understood by science. Do what you can to help yourself stay healthy, but there is never a zero risk of critical failure.