Why isn't allomancy of both Ruin and Preservation?

The reason why Allomancy is Preservation is because to 'preserve' something means exerting external power to keep it the way it is. Without a constant influx of power, things tend to decay rapidly. The sun is a constant source of power, for example, and without it, the Earth would die quickly. Therefore, it 'preserves' us.

At least, that's how I think of it :y