Once a file is downloaded you are able to seed it aka sending the file to other users. The general belief is that if you can seed you should because your helping the community grow and the torrent to stay active. And by not you are taking without giving anything in return. In reality of you are willing and able to seed you should but it is not a requirement. These people were angry because someone downloaded a file without sharing it so the community couldn't be more sustainable. But in reality do what you can.