Apple aren't stupid and lnow exactly who buys what and how to maximise their profit margins. In the case of the 8gb RAM models they will be selling these in bulk to companies more than users so it makes sense to them to save $20 per unit and put in the bare minimum.

When individuals come around they can then fleece them with a 10x markup on doubling the memory. You don't become the richest company in the world by playing fair.

(Apple are in fact being stingy though and should offer 16gb as a free upgrade to customers only buying one machine)