The misconception is that there is an absolute monetary loss for a preowned sale, but that's not strictly speaking the case, in a similar way that a pirated game would not have necessarily equated to a full-priced sale had the preowned/pirated version not been available.
The premise is relatively straightforward however. When Person A buys a game new, the store, publisher and developer each make a profit (it is in fact the only way the publisher makes money (not including DLCs/expansions and such like)). If Person A then gives or sells the game to Person B, that is now two people who have played the game but only a single sale. If the game is sold preowned by a store, the store makes another profit but the publisher and developer make nothing. This is very lucrative for stores as they can make profit by effectively selling the same product twice.
eg. Store buys game new from publisher at wholesale price: $20 (revenue for publisher)
Store sells game to customer - $30 (store gets $10 profit)
Customer brings game back to trade in - Store pays $15
Store sells game preowned - $20 ($5 profit)
So in this process the results are:
Store spends $35 and makes $50 - $15 net profit
Publisher makes $20
Customer A - Played the game, no longer owns it and is down $15
Customer B - Owns the game, spent $20
The numbers aren't accurate or reflective but illustrate the process. Publishers are basically unhappy that they are losing potential sales because someone who wants their product is getting it through an avenue (legal though it is) that they see no return for. If both customers bought the game new, they would have made $40 revenue instead of only $20. The stores obviously love preowned sales and I believe a significant proportion of profits are made from them.
I can see their argument. The physical product is irrelevant, they *should* get paid for each access to the content. But I am a firm believer that if you buy something it belongs to you...not the right to profit from the IP nor to infringe copyright, but the physical copy of it and it is thus yours to give away or sell on. It is the same with clothes, books, DVDs and CDs. Unlike with any of those items however, developers have a unique ability to entice new sales by offering "first purchasers" additional/exclusive content (see Mass Effect 2 or Dead Space 2) because of the ability to use the internet to add new features.
My only gripe, which is perhaps unfair but is aimed at the stores who profiteer like this. Sell the game yourself on eBay or something for $1 less than the store would. You get more than you would by trading in, the buyer pays less than they would from the store and the store doesn't profit twice by taking advantage of the market.