I can see the conversation that lead up to that casting:

*Bunch of CSI producers/execs sitting at a large table*

Some guy: "We should cast Justin Bieber for that role"
Exec: "You're fired"
Some guy: "No, really... we will have a huge draw in the 8-12 year old female group and people who never watch CSI but would love to see Justin Bieber die in a hail of gun fire"