Why would Antimatter behave differently via Gravity?

Confinement of antihydrogen might help provide a future answer. http://arxiv.org/abs/1104.4982

Answer

You're of course right. Experimentally established, known laws of physics - especially the equivalence principle - are enough to be certain that antimatter has the same gravitational properties - including universal attraction - as ordinary matter.

http://motls.blogspot.com/2010/09/can-antimatters-gravity-be-repulsive.html

The justification of further experiments by "tests of antimatter's gravity" is partly based on ignorance and partly on deliberate deception to get funding.