This is because you're expecting it to be an imperative language. It's not. OpenSCAD really should throw an error on compile if you try to redefine variables,

This is because you're expecting it to be an imperative language. It's not. OpenSCAD really should throw an error on compile if you try to redefine variables, but its intent is a declarative language. Once you think of it as such, most of its design choices start to make sense.