When dragging straight from the library like this onto selected (red) items, when it turns purple this actually means you are replacing one for another. This is useful for doing things like swapping fixture types, select all mac700 then drag an alpha spot from library, hover until it turns purple and they will all be exchanged. To snap fixtures to a pipe first drop them anywhere in your drawing them drag and hover on the pipe.
It is weird though that it allowed you to exchange a pipe for a fixture.