I agree that the answer is quite short, simple and possibly a bit naïve, from what we are shown in the video.
However, I think we should bear in mind that he only answered (and shortly at that) the question of why did he become a vegan at the age of twenty, and not 'Why is he a vegan today', despite the fact that the title would suggest differently.
So, the video should be changed or elaborated upon, for I am sure that Moby has substantial reasons, and many of them, for choosing to be a vegan.
In it's current form, the video portrays what I think is a simplified view of Moby (once) reasons for choosing to become a vegan.
Nothing wrong with being a vegan because you love your pet, but to my mind the matter is so infinitely more complicated than that, and I'm very sure that Moby's complete opinions on veganhood (is that a word?) would reflect something of a more complex magnitude.
Thanks for the site, and yes, I saw it on the Colbert Report. ;-)