Books are a tricky little economy, even moreso with the HP books (which almost all retailers will be selling for close to cost, some at a loss).
But the MSRP is usually fixed with the knowledge that retailers will offer discounts on them anyway (between 20 and 50%, a relatively drastic slash on the MSRP when compared to other consumer goods), in an attempt to lure more buyers into bookstores.
Not many people buy books anymore, so they have to use these kinds of tactics to get people to come into the store. The HP series is an oddity, considering that even at $100 they'd have people still lining up to buy them, but the clout of Big Box retailers (who can buy huge quantities of books at much lower costs to themselves and relative to smaller bookstores) pretty much forces independents to (reluctantly) join in on the pricing wars.