Are you saying that psychotherapy is ineffective only against those actively using MJ, but works fine in cases of practicing alcoholics or others actively using licit or illicit drugs?
The truth is that being addicted to any drug arrests emotional development and this effect is not limited to just MJ.
“Are you saying that psychotherapy is ineffective only against those actively using MJ”
I think you may misunderstand the implications for psychotherapy in the settings of use, versus abuse, versus addiction, versus intoxication with various substances.
First, psychotherapy can be ineffective for a wide variety of reasons, and these reasons may be related to drug use or not. Likewise, impediments to emotional development can occur for a wide variety of reasons unrelated to substances.
Second, many potentially addictive and abusable classes of meds are actually prescribed in psychiatry and medicine, and in the proper therapeutic doses they usually have no significant adverse effect on whether people make good use of psychotherapy or not, and in certain cases can actually facilitate it.
By extension, recreational or intermittent use and even abuse of these same sorts of substances does not entirely bring psychological maturation or the benefits of psychotherapy to a halt the way even small periodic use of MJ does. Once again, this is likely due to its slow clearance from parts of the brain where its effect is to inhibit the laying down of new emotional memories.
Full-out addictive use of daily heavy doses of intoxicants and such is, of course, quite another matter, and you are right that this would be a contraindication for psychotherapy in my practice. But the insidious effect of MJ is that it robs the effectiveness of psychotherapy even in moderate intermittent use, and of all the substances that might undermine psychotherapy it stands alone in this regard IMO.
Hope that clears that up.