You are acting as an Ultrapeer. This is good for the network. It is equivalent of being a big offerer of files except you're helping to maintain the network operationabiltity. I only wish I had a good enough bandwidth to also offer as a UP.

If you want to find out more about ultrapeer, do a search of the forum of the word leaf or ultrapeer.

If you really don't like being so generous then go to Tools>Options>Speed & tick disable up.