Maybe the feeling is: everybody nowadays knows that Mars and Venus are a) uninhabited, and b) inhospitable to human life, so audiences wont buy seeing human heroes without protective suits running about in Martian or Venusian cities having adventures with the natives. If so, I think this greatly overestimates audiences concern with scientific accuracy and/or underestimates their willingness to suspend disbelief. (After all, Avatar audiences bought this.)
On the contrary, I would think that the phrase John Carter of Mars which (even for people whove never heard of the books) promises science-fiction action-adventure is a bigger draw than John Carter, which for most audiences suggests nothing in particular. (And ditto, mutatis mutandis, for Carson of Venus.)
Another suggestion is that the studio shortened the title in order to be able to establish IP rights to the name John Carter. (They already own John Carter of Mars.) But it seems to me they could do that just by releasing a five-minute animated tie-in called John Carter, and leaving the movie with the cooler title.
Actually Id prefer the proper title, A Princess of Mars. But Id be willing to bet that some studio exec thought, Male audiences will be scared off by a film with princess in the title; theyll think its some girly rainbow thing.