The two illustrations below show the typical spoiler location and the lift distribution of a wing. For comparison a third picture shows the location of spoilers on a glider.
It appears the airliner spoiler is located so it destroys very little lift but would really only contribute to drag - is this really the primary contribution spoilers have to the aerodynamics of an airliner or is there more happening here?
Typical location and size of airliner spoilers. - source
Typical lift distribution of wing. Red references have been added to show typical airliner spoiler. - source
For comparison, a glider spoiler located at the center of lift - source