Assuming you are talking about using chumming in the fishing sense...
Because it lures fish in....
Think of kokanee on Kawkawa lake, they feed on krill.
Chum the water with alot of krill, school comes in to feed (you are also using krill) and you whipe out the whole school.
It is there to give fish a chance just like natural bait cannot be used on some rivers, it gives fish a sporting chance.
Do you think it is ok to bait a bear if you are hunting?
If you are just referring to feeding fish without fishing while doing so....
It will give fish a reliancy on humans. Its th reason why they have signs up all over provincial parks "do not feed the animals." They want the animals to live on their own.
Either way it is not good for the fish.
Just let nature do its own thing.