Exactly - it is pure fantasy, based on a comic book.
Now, if an actual "zombie virus" existed in the real world...yeah, we could complain about how a TV show portrays it "unrealistically". But would we have time to do so if such a thing played out for real? I have a feeling 2-3 AVSers would, right up until they died & resurrected hungry for flesh.
So if hacking off a limb early to avoid zombification after a bite to an extremity worked out for the folks in the comics, I'm also cool with it working on the TV show.