This policy isn't restricted to ATI however, but exchanges for same product, etc...can be pretty common.
This happened with a friend of mine, and an Intel proc. To make a long story short, he bought a Pentium II 266, and Intel had given no indication through their dealer channels that the procs didn't support ECC...
My friend, after having bought the system, found this information out online, by checking his CPU's core reference number against the technical specifications sheet for it on the Intel developer site. He went back to the store and inquired this, as he wanted ECC.
The people at the computer store were surprised, though didn't doubt him (and of course the link was on their site). Apperently, Intel never told their developers that this was the case, first they heard of it. So Fred went around to various other OEM shops in town, and asked if they heard of it, and all indicated "no we haven't". He then showed them the documentation my friend found, and informed them "this is so, though the information has not been made available on the dealer channels".
In the end, he pulled in a few favors with people he knew, who worked at Intel, and arranged a meeting between the 2 companies to discuss this matter with them, and also push for them to replace the processor for my friend on grounds that the CPU wasn't as advertized to the dealers themselves...
When he finally got a meeting setup, and had gotten to speak with these people, in person, they first inquired as to why anyone would want ECC. Fred (himself a computer engineer since the 1950s, who only was working in the OEM business to help a lifelong friend who acquired the shop and became business owner several years back); not only told them, but also how Intel could make their ECC implementation better, why they should design it differently.
The Intel reps weren't expecting someone knowledgeable enough to tell them how they can make their products better, so out went any possible counter argument
In the end, they would not replace the processor, but much like ATI, only did a trade up against the, then current market value to his PII 266, and applied it towards a PII 300 (none of which had faulty ECC).
As said, this practice isn't unique to ATI, but applies to other manufacturers also. As to my friend, the store could not justify asking him for more money for the new CPU, just gave it to him, with no comment on the matter. However, Fred had told me that this was the case, and that they wrote it up as a business expense incurred onto themselves. Discussing it with the owner, he could not see a reason to pass this cost onto the consumer, so never told him...
Intel at least, has made use of this tradeup value also...