Originally Posted by IronManlet
It has to do with the physiology of the movement: your arms are doing the pushing. The pectoral muscles' primary purpose is to pull the arms down and across your torso. The way your pectorals are stressed during the movement is towards the bottom position; when they are in a full stretch.
It is impossible to push your arms upwards from a pronated position without pectoral involvement, hence they are abductors for your arms. However, your chest is not actually lifting anything; your Triceps and shoulders are. This is why Tricep strength is so often stressed when training your Bench.
If you really want to stress your chest muscles, try doing a variant that gives them as full of a stretch as possible in the bottom position. You will get a big chest, but that's not because your chest was doing a lot of pushing.
I'm glad you're a well read lifter, but the body mechanics lesson was a little over the top for the point I'm trying to get across. Bottom line, you can take one compound exercise and make it do different things for your body, and safely too.