By James Hamill / For the Journal   The tax law is used to advance, or discourage, many social and economic objectives. This is not a bad thing, and has suited both sides of the political aisle in different ways. It is often difficult to find the target audience to subsidize an activity. A tax subsidy allows the audience to identify itself. Tax credits can be refundable or not. If refundable, even those who might not otherwise file a tax return can benefit by the simple act of filing. For any tax subsidy, some support it and others not. This...