I've been a cop for almost 20 years. I don't recall ever being trained to disregard statements that exonerate people. In fact those are generally called "alibis". They either check out, or they don't. If they do, then we know the person is telling the truth. As far as the "DA" and what he/she says, it's kind of irrelevant. Police are required to give all available evidence to the defense, including all statements, exculpatory statements included. It's not the DA's job to present a defense for a defendant, it's the defense's job. The police, if they're doing what they're supposed to, an innocent person who talks to them won't get charged in the first place. In my neck of the woods, the "DA" doesn't make charges, we do. They can drop them, lower them, or modify them once we do, but the decision to charge is ours.
Completely contrary to what America's Patriot posted, police promotions are not based on criminal convictions. Arrest numbers are part of what is looked at to consider if a person is doing their job and worthy of promotion, but not convictions. I couldn't tell you right now if my life depended on it how many cases I made resulted in convictions, because it's not my job to secure convictions, that falls to the prosecutor. Cases that are very good often get dismissed, or plead down, to clear the docket... my bosses only care that I make a good case, what is done with the case once it leaves my hands and goes to the prosecutor is not for me (or my bosses) to worry about. Literally nobody in my entire chain of command has ever commented on the amount of convictions I do, or do not, get. It's just not done. Period. Any cop with more than a week on the job will tell you that arrests are the only numbers that count in that regard. Once the arrest is made, the case is closed. What happens after that is of little consequence to the officer, as long as he didn't goon up and do something so egregious that comes out on the stand, like beat the suspect or violate his rights. As a detective my job is to clear cases, NOT make arrests. If I determine a case is unfounded, that's a clearance. If the victim refuses to cooperate, that's a clearance. I'm after the truth of what happened, as best as I can find it. Sometimes that results in charges, sometimes it doesn't. I get paid the same either way and promotions don't hinge on it at all. I've never met or heard from or of any cop who got promoted based on the number of convictions his cases generated. Never.
Let me tell you all a story about why it's a bad idea to take blanket legal advice from someone on the internet, even a professor. About 2 or 3 years ago a teenage girl at our local high school reported she was raped and sodomized by her ex-boyfriend who was a fellow student. The security camera footage from the school contradicted some parts of her story, but not enough for us to say she was definitely lying. Supposedly he followed her into the girls' bathroom and committed the act there. When we got in touch with his mother (he was 16) she started talking about how she might need a lawyer. I convinced her to bring him in and get his side of it. He was able to exonerate himself by telling the entire story, which made sense in light of the camera footage... turns out she sent him text messages asking him in very graphic terms if he was interested in getting it on. He, being a 16 year old boy, elected to fulfill her request. Now had he "lawyered up" we would have had no choice, due to the pressure put on us by the public who would inevitably hear of a "rape" at their school, to charge him based on the girl's statements alone... the word of the alleged victim does count as probable cause and if we can't disprove the "victim's" account then we have little choice in the matter. Now, could his mother have spent a few thousand dollars and won her son's freedom had we charged him? Probably. But by disregarding the idea of "never talk to the police", she saved her family major embarrassment, a HUGE amount of time and money, and he was cleared in less than half an hour. And the girl, who was a little loony in the head, was booted from school but not charged, due to her age. Her son went on to have a banner year in football and wrestling and last I heard was going to college. I don't think anybody would believe that would be the case if he was charged with rape, even if he ultimately won the case in court, the damage would have been unfathomable. You could easily cite this example, among many others, to make the complete opposite case that you should talk to police. The point is that the phrase "never say never" has merit here. By all means, if they police are wanting to talk to you and you ACTUALLY DID DO IT, then don't talk to them.
If every person, witnesses included, elected to "never" talk to the police, society would be an even nastier place than it already is. The word "never" is pretty broad. I can't see what good not telling the police what kind of car you saw fleeing a bank robbery, for example, would be a good idea. I get it from the standpoint of if they are questioning you about something that they think YOU did, but not as a witness. The bottom line is that the decision to talk, or not, should be based on the unique circumstances you find yourself in, not a video on Youtube.