Unfortunately, running a public SMP server just requires certain sacrifices in order for it to work out for everyone. Restrictions on dumping water/lava outside of claims, fire spread being disabled, the number of mobs which can be spawned into a chunk/area; these are all things which are necessary to keep the server free from grief, to keep the server enjoyable for all.
I dislike mob grinders, I think they're a terribly cheaty way to play the game. But they are a part of many people's Minecraft, I do not look to dictate how you play your game. Spawning 600 zombies from a single spawner, whilst you sit AFK, affects everyone who plays on the server. It causes lag, it reduces the number of mobs which can be spawned in the rest of the world. Animals farms also cause lag when there are hundreds of animals in the area. As it is, you can get near 70 passive mobs in a chunk under the limits imposed. This is a comfortably large number of mobs, I understand some like to cram animals into tiny pens... and.. well.. that's just cruel

At spawners, you can get a fairly steady stream of mobs to be killed. Idling at them gives you no benefit, but you can actively grind at them for drops and xp. Other types of mob farms are harder to automatically restrict without affecting natural spawn rates, and they have on occasion caused problems.
If you want to hoard larger number of animals, then you will need to spread them out. It's good for the server, and it's good for players who may be visiting your area. Some people's computers just cannot handle large numbers of mobs.