RLSH700, you answer your own question.
The whole thing is it begs the question, why would you bother to buy a Challenger for a police car over a Charger. They will weigh about the same, they will use the same powertrains (with the exception of the Challenger getting a manual, which police prefer automatics on the column for their computers and equipment), a similar suspension, etc. You will end up with a more inconvenient path to put stubborn criminals, less storage room, with no real advantages. If the issue is the police need more power than the 5.7L, it would make more sense to just offer the 6.1L in the Charger. Something to think about, the Magnum police package was not a very hot seller, and that was technically a more practical choice for the police as it had the performance of the Charger with the room for a K-9 unit cage all in one package, and/or more storage space. If the police passed up on that, why would they go with less convenience and space?
They probably went with the sedans because of some silly stigma or concern over IMAGE. That is the exact same reason they would opt for a Challenger, simply because it'll look tough. I'm not saying they'll have ton's of 'em...but I'd be surprised if there weren't more than a few across the nations PDs or state police.
You're point is absolutely valid, the most practical vehicle would have been the Magnum, but they didn't opt for it.
Think about it, why would they purchase HD's over Honda's or some other more reliable bike? They get Harley's because of image I would venture.