POM is end gamer gear. As endgamers get to be more powerful, their ability to farm L75 bosses for POMs increases. Yesterday all-OP raid is today's good group is tomorrow small group. Think Zanka - it used to require OP raid, now a small decent DPS group can down it. So, since POMs are technically a lot easier to obtain, they have to devalue them. That is done both via reduction of number of POMs you get and via increased POM cost of gear.
That is same principle behind memento price increases. More advanced gear means more mementos.
So, why is that not done same way with shells? Well, because your ability to farm shells is not increased as you level up, unlike your ability to farm POMs and mementos. You get same shells at L70 as you get at L82, thus L70 gear at the time when 70 is cap is worth the same as L82 gear when 82 is cap.
-- Rustyx --- 92R / 92S / 92M on Reni (Guild KnightShift). Yes, running the new FOTM R/M, cause I am not elf enough to be WD/S.
Oh, and people who have more than 3 classes are clinically insane.