That's kind of funny, I never noticed that video cards have listed memory requirements.
Anyway, it's bollocks. You could install the video card with 128 MB of RAM and be fine (aside from, you know, the crippling effects of having only 128 MB of RAM). AMD probably lists a minimum requirement so users who have crap systems don't complain about how the video card didn't make things any faster.
Protecting themselves from loud, angry, stupid consumers who think they're computer savvy.