http://www.washingtonpost.com/news/opin ... -immunity/
I would pick out this paragraph:
It was an absurd argument that would have essentially rendered the “investigation” exception meaningless. The only time a prosecutor could be held liable for manufacturing evidence would be if a different prosecutor then used that evidence at trial. Nevertheless, during oral arguments, it seemed to have some resonance with at least a few of the Supreme Court justices. But most of the justices seemed skeptical, and it appeared as if the court would affirm the “investigation” exception — prosecutors who knowingly manufacture evidence that results in the conviction of an innocent person shouldn’t be shielded from lawsuits. (Read the clause after the dash again. The idea that this would even be up for discussion shows just how far down the rabbit hole we’ve fallen.)
US prosecutors see themselves as "only doing their job". And this "job" consists of suborning perjury, hiding evidence, threatening defense witnesses, manipulating the press, whatever is need to obtain a conviction from the jury, and a vote from the public at the next election. They have absolute immunity from prosecution, so provided they are careful, there is no problem carrying on in this way. This is the system. Levy is simply a typical example.