Because to pay off the debt, Apple first would have to move the cash back to the US, at which time by the current tax law, it would be taxed by 35%. So, to pay

Because to pay off the debt, Apple first would have to move the cash back to the US, at which time by the current tax law, it would be taxed by 35%. So, to pay back $150B, they would have to bring back about $230B, paying $80B in taxes.