As noted in another comment of mine, they didn't blame murder on him, but instead prosecuted more technical, less interpretable crimes.

I agree that murder may be an excessive charge given lack of intent, yet in the United States murder is often broadened to include committing a serious crime (felony) where there is a clear danger of killing someone, and it resulted in death. Then again, I would argue that this goes well beyond just a 'prank call' in that there is intent to specifically victimise someone by deceiving armed agents.