I can understand being frustrated that your OS is no longer supported. And you're right, AMD's download page was misleading as it made it seem as though it were supported. However, as others have stated, you'd be in the same boat if you had bought a GTX970, as the 900 series isn't supported on 32 bit XP (and their driver page reflects this, it shows no available drivers for that card/os combo).
But to be fair to AMD/NV, I think it's a reasonable expectation for someone who is a moderator on a tech forum, and an avid DC enthusiast to realize that a 32 bit OS that old, for which Microsoft themselves have stopped supporting, to see driver compatibility issues coming and to check before buying new hardware for use on legacy systems.
Also, it's not $150 for an OS, it's $100 and often less. I think springing for a modern OS every dozen years or so isn't a big ask for someone as involved in the tech community as you. Or how long do you think is appropriate for hardware vendors to support OS's that are 3 going on 4 generations old AND 32 bit?
While continued support is nice for the .00001% of users that still need it, in my opinion it is no longer reasonable to expect it on new products.