Approximately 7 states require Medical Doctors to have liability insurance. In fact, lots of doctors choose to take their chances. Most hospitals require their doctors to have this insurance. As for the title of psychotherapist which is a protected title in most, if not all the US. I am willing to bet that very few states require psychotherapist to protect themselves.
Lack of malpractice insurance should not stop anyone from filing a law suit - finding a lawyer to take the case might be difficult - psychotherapist and MD's personal property, future income, etc can be part of the win package.

I don't believe sexual abuse would be excluded from any of this as others have said, though it is poor consultation for such an egregious act.

In the UK all psychotherapist are required to have liability insurance.