Why Don't Prosecutors Get Brady?

The United States Supreme Court per Justice Stevens has found that a Tennessee prosecutor withheld exculpatory evidence in a case where the defendant has now been on death row for 27 years.

Bell v. Cone, http://www.supremecourtus.gov/opinions/08pdf/07-1114.pdf

The defense argued at sentencing that the death penalty was not appropriate because of the defendant's long history of drug abuse, which may have started during his otherwise honorable military service in Vietnam, diminished his capacity to appreciate the wrongfulness of his conduct. The prosecutor argued that the defendant was a drug seller, not a drug user, and that his claim of diminished capacity was "baloney."

The prosecutor, however, had suppressed evidence that witnesses, who had seen the defendant before and after the murders, had described the defendant as "wild eyed" and as acting "real weird" as if he were "drunk or high." There was also a police report describing the defendant's demeanor at the time of arrest as "frenzied" and "agitated." And, in direct contradiction to the prosecutor's argument to the jury, there were multiple police bulletins describing the defendant as a drug user.

The opinion is mainly about whether the defendant was barred on procedural grounds from even raising the claim in federal court. The Court finds he is not barred and remands to the lower courts. That part is of interest to federal habeas nerds. What I found most compelling is that the prosecutor told the jury that the defendant was lying about his drug addiction when the prosecutor knew different. This happens, I am sure, much, much more than we know. Why don't prosecutors see that's wrong? Why don't they see that's against the law? I mean, Brady was decided in 1963! They've had time to read it carefully by now. What am I missing here?