Heterolytic bond fission means, when the bond breaks the bonding electrons are not shared equally, but one species takes u both the electrons gaining negative charge while leaving positive charge on the other. When the bond is polar i.e., the bond is formed between to atoms of different electronegativities the electron density will be more with the one more electronegative out of the two. In this case the bond is between C and Br. Out of which Br is more electronegative, so when heterolytic bond fission occurs both the bonding electrons will be taken by Br, leaving positive charge on carbon.