Answer:
It is believed that emus cannot walk backwards, as no evidence exists to the contrary. That is why, together with the kangaroo (which also cannot walk backwards), the emu is on the Australian coat-of-arms, symbolising progression forwards.
Whether or not emus can or cannot walk backwards has never been definitively proven, as the phenomenon of them walking backwards has been neither observed nor recorded. It is believed that the joints in their knees make it impossible for them to walk backwards.