Here's the problem I think the police are considering: sometimes evidence that would be otherwise convincing gets judged inadmissible in court because of some technicality or other, and not supposed to be taken into consideration (I didn't realize this was even that common until I served jury duty and saw this happen). So even if they're going after someone for a greater charge, the evidence that proves it can't get used, and so they can only go with a lesser charge.

But yeah, I'm not really clear this is entirely a good reason (or the right reason) to oppose the question. Just something that I was considering when I was weighing the relative merits of both sides.