"All police aircraft should have them fitted, and cost shouldn't be a reason not to fit them. "
It's easy to spend somebody's money especially when it's nobody's money, in other words- taxes.
Organize a non-profit and solicit voluntary contributions to equip all copper choppers with black boxes, or just buy them yourself, if you want. I can think of a lot of other things that law enforcement needs before black boxes, stuff that would make a difference in the present and not a hypothetical future.