For some reason screaming vocals have been attributed to metal by many people, when it is actually a punk trait. The harsh vocals that are found in metal are usually just growls or shrieks, not actual screaming as such. People just dont seem to realize that metal is a genre defined by the instrumentation, vocal style does not matter. You can scream all you want, but unless you use heavily distorted guitars, and just a general heaviness to the music then it isnt metal.
The aggression of hardcore, screamo etc is found in the vocals, while in metal it is the instrumental sound. Alot of people assume that vocal aggression automatically means its metal.