Exactly. Customers prefer aisle and window seats so airlines charge more for them (or, if you prefer, offer a discount for middle seats). If they don't sell all of them at the higher price, they make them available at a lower price.[QUOTE="let's block all aisle seats even in back rows so that people that really want them will be forced to pay extra, and then we'll unblock them for other less desperate people checking in at last minute who would not have paid anyway).
All products work the same way. That's why a toy costs less after Christmas (if it's available) than it does before.