I think the plaintiffs feel that they deserve *full* support, rather than "Well, it'll run." While OS X is certified to run on a whole bunch of G3 models (don't remember the list, but it's on Apple's site), the improvements (especially the accelerated video for Aqua) are aimed at the newer models, particularly the G4s.
This seems to be a nuisance suit, but IANAL.
>Hey if I have a Pentium III system that don't run XP, can I sue Microsoft? ;)
If MS advertised the XP supported your hardware, the answer is apparently yes.