Depending on how 'out-there' you want to go, I can think of a few good reasons for mecha police, and for rookie cops to be kitted out as such.
If your mecha operate on a direct neural interface, it may be that most 'norms can't operate them very well. Only a handful of individuals can make full use of mecha - many of them people who grew up playing neural interface games, which have only been available for the last 15-20 years. Essentially, the neural interface games 'train' the developing nervous system of a child to tolerate this kind of interface, but only the handful of adults who could have piloted the mecha anyway can play the games without having done so as kids.
Unfortunately, this has led to a younger generation that largely considers itself an evolved form of humanity. Many of them see no need or use for the 'archies' (slang for the older generation, derived from the word archaic) who can't employ modern technology. The result: Youth Movement. Half terrorist organization, half l33t hacker club, all neural interface anarchy. Youth Movement was originally considered a harmless development, but in recent years its members have begun using their abilities to terrorize 'archies.'
The police can't combat Youth Movement terrorism without using mecha of their own, and most (all?) of their potential pilots are rookies of the same generation as the enemy, raised on neural interfaces and capable of going toe to toe with criminal mecha.
