It comes down to profit lines. Sprint and its in-direct retailers have more of an over head than an online store with the computer doing all the work. @nchargreed, there is actually a formula for profit per sale (different for upgrade or new account) that deducts the value of the phone. at $80.00 (tiger direct), I could mark down a galaxy for a new activation at a 50% profit cut. On an upgrade I would be taking $45 loss at that price. Not sure how or why some online places offer the same price for both, but I do see Best Buy and Radio Shack offering different prices.
Most In-Direct retailers would work with you on pricing, where a corporate store is pretty strick about pricing.
And you have been lucky about 2nd party exchanges. Sprint leaves the exchanges up to the manager at Corporate location's discression. They can and have refused to take exchanges on 2nd party devices. If you bought Nike shoes from Foot Locker and tried to return them to a Nike store, they can refuse the return or exchange. Unlike Nike however, Sprint doesn't produce the phones. This is not refering to Sprint insurance claims, which is actually through a different company, and manufacturing defects.
As far as this Urban Legend you speak of... I'd gladly show you my stores profit per device except I like having a job. If you go to an in-direct retailer (non-corporate Sprint location but with Sprint branding) and try to haggle with them, some of the associates (whether they are supposed to or not) will show you / make reference to this.
Finally, this is the same reason why ALL RETAILERS are not pushing the damn iPhone. Not only is it over hyped because of the fruit on the back, but the cost of the device takes away from the sale. That shouldn't be anyone's reason for not selling it but still.