In an article called, "Being An Effective Expert Witness," Resnick writes:

In fact, the United States Supreme Court has commented on the "pivotal role that psychiatry has come to play in criminal proceedings."²

The reference is from Ake v. Oklahoma.  He doesn't say it, but it sounds like he's saying that even the Supreme Court recognizes that psychiatry has much to say about criminal behavior.   See how it's reference 2?  It's early in the article, it's in the introduction-- it represents our foundation, our "knowns," before it delves into new stuff.  No one investigates the "knowns" because-- they're known.   It tacitly endorses the notion "everyone knows psychiatry has much to say about criminal behavior" and uses the Supreme Court (ipse dixit!) as a reference.

In psychiatry, you should always be most suspicious of the Introduction, and the references cited there:

Last, we inquire into the probable value of the psychiatric assistance sought, and the risk of error in the proceeding if such assistance is not offered. We begin by considering the pivotal role that psychiatry has come to play in criminal proceedings...

The Court (Justice Marshall) then goes on to explain the State's obligation to provide psychiatric expert services to those who can't afford it when it is relevant to "an adequate defense," namely, "defendant's mental condition relevant to his criminal culpability and to the punishment he might suffer."  They explain what such an expert would do (gather facts, draw plausible conclusions about mental states, explain it all to the jury, etc), but then:

Psychiatry is not, however, an exact science, and psychiatrists disagree widely and frequently... Perhaps because there often is no single, accurate psychiatric conclusion on legal insanity in a given case, juries remain the primary factfinders on this issue, and they must resolve differences in opinion within the psychiatric profession on the basis of the evidence offered by each party.

So: 1) psychiatrists have a limited expertise in criminal matters-- e.g. specifically insanity; 2) juries still must decide, not just because that's the way we set up the legal system, but also because psychiatry is so inexact.  It is totally reasonable for juries to disagree with psychiatrists.

In so saying, we neither approve nor disapprove the widespread reliance on psychiatrists but instead recognize the unfairness of [not offering psychiatric experts to the indigent] in light of the evolving practice [of offering them to everyone else.]

It's also worth mentioning that the sole dissent in this case was Rehnquist, who wanted the obligation to provide psychiatric experts to the indigent limited to an independent psychiatric evaluation-- not an expert witness.

Finally: the obligation of the State to provide psychiatric experts exists only if the defendant's "sanity at the time of the offense" is shown to be at issue.  If the defense doesn't or can't contest the defendant's sanity/mental state, then the State isn't obligated to provide experts. And merely saying it isn't enough-- it has to be above a reasonable doubt.  In this case, the defense did not do this, but be aware that the issue of sanity has to be raised before experts get involved-- i.e. it has to be apparent to a layman looking at the history. 

The evidence of the brutal murders perpetrated on the victims [by Ake in the original case], and of the month-long crime spree following the murders, would not seem to raise any question of sanity unless one were to adopt the dubious doctrine that no one in his right mind would commit a murder.

Looking insane isn't enough.  Doing really bad things, like setting out with a coworker, tricking your way into a family's home, tying them (a Reverend and his wife) up and then shooting them, raping the 12 year old daughter (and failing) isn't enough. The problem with forensic psychiatry education is the same as for general psychiatry: little time is spent re-evaluating the principles. I'm not blaming Resnick-- clearly, he knows his business well, and his forensic program is well respected.  The problem is systemic to psychiatry, and probably to all social sciences.  Psychiatrists are trained on how to do things, not whether they are supposed to be doing them, or whether they have the knowledge and ability to do them.  Criminality and psychiatry may drive the same roads sometimes, but they don't use the same car.