The issue is that both groups derive from a single block chain at some point, so everyone who held BTC before that split can spend them on both forks, whereas t

The issue is that both groups derive from a single block chain at some point, so everyone who held BTC before that split can spend them on both forks, whereas those who receive them or mine them after the split only hold the currency on one fork. Additionally, if the split of hash power is highly asymmetric, then the difficulty of the majority fork will become much higher than that of the minority fork, over time. This opens the minority fork up to 51% attacks. Say the split is 90/10, then just 15% of the miners from the majority could team up and switch back to the minority fork to wreak havoc.