I was once low level due to weather one evening as we were diverting. PF had eyes outside while I, PNF, had eyes on map tracking control zones and obstacles.
Sounds to me that the coppers don't want to spend the money for two qualified pilots up front -- in an environment that demands it.
The human factors side of somebody somewhat qualified next to me who can assist to an unofficial (here I mean the CAA or equivalent national aeronautical authority) standard is insidious.
That said, I have not heard of any police helicopters striking obstacles; so, maybe they're doing it right. Local area knowledge has to count for something.