Well, let's look at it from another perspective: If the ETH you lent out was shorted and someone thus actually bought the ETH from the shorter, it is rightfully his and he should get it. Therefore, if you argue that you should also get the ETC, then Poloniex either has to somehow duplicate the ETC for those ETH (not possible) or punish the shorter and make him also pay back your ETC, in addition to your ETH. I think that between the two choices, Poloniex chose the best one. Although I do understand that whatever option Poloniex would have chosen, one side would always feel robbed.