Ceiling lamp wiring problem
Pictures of the inside of the fixture box and switch box would be nice to confirm this, but based on your description it sounds like the installer wanted to be able to put a ceiling fan in this location.
In my parents' house they have a lot of ceiling fans wired this way - the fan is always on (and controlled by a pull cord) while the light is controlled by the wall switch.
The purpose of this arrangement is to make it easy to turn the light on and off while leaving the fan alone.

Since you are putting in only a light fixture, you should cap the black wire with a wire nut. For safety to prevent the nut from coming off you can tape the nut onto the wire with electrical tape (after screwing it properly to the wire; do not rely only on the tape to hold it there).

After capping the black, you should be able to hook up your fixture like this:


fixture black to house red
fixture white to house white
fixture green to house bare ground