It's controversial because there is some misinformation out there. Also, some people want to believe that if a vegan diet is a natural diet meant for humans, that should mean we don't need to take any supplements (more on that later).
The controversy is mostly regarding some supposed plant sources of B12, like seaweeds. These have been found in studies to contain B12 analogs, which appear to be B12, but are not assimilated by the body. (Though some people will claim them as sources of B12, this is both misleading and dangerous.)
B12 is created only by bacteria, which is where animals get in in their systems in the first place.
As for the "if veganism is a natural state, then why do we need B12?" argument: Humans used to be able to get it from bacteria remaining on fresh-picked and lightly rinsed produce, or from drinking water from streams, but since our water is now chlorinated and processed, and our foods are the same, this is not a current source of B12.
The best thing to do as a vegan is to use a B12 supplement, which is created from bacteria in a culture. I like the Methylcobalamin spray by PUre Advantage, which www.veganessentials.com
carries. Methylcobalamin is a more effective form of B12 than cyanacobalamin (which is broken down in to methylcobalamin in the body), and is what I've also used to reverse neuropathy in my diabetic cat.