According to a Federal Evidence Rule hearsay exception (FRE 803(8)), police investigatory reports cannot be admitted for use against a defendant in a criminal trial. Basically, it would be bad if the prosecutors could rely solely on a report by a cop who isn't even in court to convict someone. (And 6th Amendment right to confront your accuser.)
My Evidence prof picked through the policy reasons on this by playing devil's advocate. In the case at hand, a defendant was prosecuted for selling heroin. My prof suggested that it might be better for a cop to avoid testifying on the stand because a defendant has a motive to off the cop. To tease this out of us, he asked us:
"Are dope dealers just friendly peddlers of pleasure?"
[Laughter from the class.]
"Or are they like Like Westlaw and Lexis, handing out free samples to law students so that you’re hooked forever?"