First off, the cameras are always hooded, to reduce sun glare and reduce the amount of rainwater that gets on them. Rain contains a surprising amount of dirt the drops pick up as they fall through the atmosphere, and you get the same effect on a camera lens as you do on your house windows.
Now hit that with a paintball and the effect is quite a lot worse. A single shot to any part of the lens or hood area is likely to completely coat the entire lens with a thick enough layer of (water-soluable) paint to make 100% of the photos taken with the camera unusable. (can't get a plate)
The rain hood will protect against rain washing the paint away. Although after a few days, the paint will have dried up enough that it will require a little scrubbing action to remove it - ordinary rain won't do the job at that point.
This method has many advantages and few disadvantages compared to other options. Firstly, it's unlikely to cause physical damage to the camera, which will be useful if you get caught. If you and four other citizens start balling the cameras, and you get caught, you will likely be judged responsible for 100% of the cameras. If you're taking them out with
Ammo is a lot cheaper. Shots can be a lot quieter. Easier to visually identify already "serviced" cameras. And odds are you'd be charged with vandalism rather than destruction of public property, since nothing permanent was done.