Mainly this thread was inspired by my shopping on the weekend where FABs were all over the place. They bug me as much this time around as they did last movie, hogging shelf space and seeming not to move. The robot vs vehicle mode seem another plausible reason to me. Most times deluxes are on sale, FABs are too, so it can't just be price.
A good suggestion that Golden Phoenix made to me was that FABs might sell better if they were scout/activator sized. I think I'd buy FABs at the size/price point.