I would disagree they are opposed anymore so than any other kind of clinical work. In fact it is quite common for forensic psychiatrists to be psychotherapists, particularly in places like NYC, Boston, SF etc. Some people even focus on things like sexual boundary violations in psychotherapy, for which the expert witness should be a psychotherapist. Others provide psychoanalytic consulting services to corporations or other organizational based services rooted in psychoanalytic approaches. There is also a subspecialty of forensic psychotherapy (it is recognized as a branch of psychiatry in the UK - there is a training program at the Tavistock) which a small cadre of psychiatrists are interested in and involves the psychoanalytic treatment of violent (including sexually violent) offenders. These are typically non-psychopathic personality disordered individuals. I'm not really sure it works, but it's nice work if you can get it. Also psychotherapy and forensic evaluations are quite detailed explorations of the individual's life. You still have to (or ought to) respond empathically to forensic evaluees, and in my experience good psychotherapy skills are essential to getting people to open up, and I often find I can't help myself in providing some sort of intervention even though I am not in a treatment role (the temptation to reassure, praise, confront, or interpret sometimes proves too much!)
Also I hate repeating myself, but there is this myth on this forum that a forensic fellowship is necessary to offer forensic services. This is completely untrue. The overwhelming majority of forensic services are provided by psychiatrists who are neither fellowship-trained or boarded in psychiatry. Some of the lower quality fellowships are probably a waste of time. "Are you boarded in forensic psychiatry?" said no attorney ever (unless you put it on your CV...) You are being retained for your expertise in clinical psychiatry in the same way any other subject matter expert or physician expert witness is. I'm not saying there aren't advantages of doing a forensic fellowship if it doesnt kill you, but it is not necessary.