"How is it that law enforcement can fly at night, while fire is grounded?"
There many reasons why most firefighting agencies do not drop at night and all are related to safety.
Water drops work when supporting the crews punching in hose or cutting line. Water is very heavy and at night people are hard to see. I have been a couple times and it can injure or at worst, kill.
Dropping water means you have to fly low, in canyons, around mountains and such, not making circles at a comfortable altitude like a PD bird.
On fires, copters are called in from where ever they can be found. While a few local pilots may be intimately familiar with local terrain , the folks from hundreds of miles away most likely are not. If you ask ( which I have) folks who do drop at night, they will tell you it is done only in limited circumstances and the NVGs are used in transit, not on the drops.
Things like power lines are hard to see.
Remember when you see LA City dropping at night, there is a lot of city glow ( I live here in LA and sometimes night is like day) and most often their firefighters are nowhere near where the drops are being made ( I never seem to see those guys going direct!).
One last thing we must take into account, the whole night dropping issue is being used by local media and those without any real knowledge of wildland firefighting or air operations in wildland firefighting (San Diego City) for sensational stories and a grab for power. CDF has been the victim of an ongoing joint attack perpetuated almost daily by the San Diego Union Tribune and the San Diego City Fire Department who are using the night dropping issue as propaganda to meet their goals. We all know dropping anything at anytime of day in the conditions we face last Fall will be token at best and not stop a wind driven fire front. The above mentioned, organizations are providing a dis-service to the public with their mis-information.
Monitor 151.220 this summer, I am sure it will be colorful.