I think the purists believe that because no Commodore Amigas came out with anything other than a 680x0, therefore it needs to be using a 680x0 to be an Amiga.
Personally, I don't understand this. The fact that an Amiga can use a PPC is testament to the versatility of the hardware and the software, and should be celebrated, not scorned. Just because it wasn't officially done by CBM doesn't mean it's not somehow "Amiga", IMHO.