After watching the movie, I understand. It wasn't about him being "of Mars", it was about him becoming it. The title screen at the beginning which mirrored the one at the end showed that. I actually appreciated it after that.greenpenguino said:
I don't get why this wasn't called 'John Carter of Mars' anyways, stupid disney.
And apparently all of the marketing, including the name change, was done by the director, Andrew Stanton, not Disney.