Everyone has the ability to terminate uploads. In the 'Monitor' tab, you can click on a file and cancel the upload.

Why do people do this?

I used to do it if someone was being greedy. For example, if I saw a certain IP address over and over again.

Other people do it because they think that they can determine whether you are sharing files or not. Many people do not like to give their files to 'freeloaders'. The thing is, there is no foolproof way to determine if someone is sharing. If someone is downloading a file from you, you may or may not be able to use the 'browse host' feature to see what files they are sharing. Do you have chat enabled? The chat function doesn't work in current versions of Limewire, but it does supposedly help the 'browse host' feature work better.

What makes you think your downloads are being intentionally terminated? Perhaps someone is done using their computer and is simply turning it off. Or maybe they have dial-up and need to use the telephone. Or maybe they downloaded a virus. Maybe they need to reboot for whatever reason. Not everyone is constantly connected to the network.

Hope this helps

Dan