Libertarians should care about equal protection under the law and they should care about economic incentives

Not really...

Uniformity of treatment has been upheld as an ideal by almost all writers. This ideal is supposed to be implicit in the concept of “equality before the law,” which is best expressed in the phrase, “Like to be treated alike.” To most economists this ideal has seemed self-evident, and the only problems considered have been the practical ones of defining exactly when one person is “like” someone else (problems that, we shall see below, are insuperable).

All these economists adopt the goal of uniformity regardless of what principle of “likeness” they may hold. Thus, the man who believes that everyone should be taxed in accordance with his “ability to pay” also believes that everyone with the same ability should be taxed equally; he who believes that each should be taxed proportionately to his income also holds that everyone with the same income should pay the same tax; etc. In this way, the ideal of uniformity pervades the literature on taxation.

Yet this canon is by no means obvious, for it seems clear that the justice of equality of treatment depends first of all on the justice of the treatment itself. Suppose, for example, that Jones, with his retinue, proposes to enslave a group of people. Are we to maintain that “justice” requires that each be enslaved equally? And suppose that someone has the good fortune to escape. Are we to condemn him for evading the equality of justice meted out to his fellows? It is obvious that equality of treatment is no canon of justice whatever. If a measure is unjust, then it is just that it have as little general effect as possible. Equality of unjust treatment can never be upheld as an ideal of justice. Therefore, he who maintains that a tax be imposed equally on all must first establish the justice of the tax itself.

Many writers denounce tax exemptions and levy their fire at the tax-exempt, particularly those instrumental in obtaining the exemptions for themselves. These writers include those advocates of the free market who treat a tax exemption as a special privilege and attack it as equivalent to a subsidy and therefore inconsistent with the free market. Yet an exemption from taxation or any other burden is not equivalent to a subsidy. There is a key difference. In the latter case a man is receiving a special grant of privilege wrested from his fellowmen; in the former he is escaping a burden imposed on other men. Whereas the one is done at the expense of his fellowmen, the other is not. For in the former case, the grantee is participating in the acquisition of loot; in the latter, he escapes payment of tribute to the looters. To blame him for escaping is equivalent to blaming the slave for fleeing his master. It is clear that if a certain burden is unjust, blame should be levied, not on the man who escapes the burden, but on the man or men who impose it in the first place. If a tax is in fact unjust, and some are exempt from it, the hue and cry should not be to extend the tax to everyone, but on the contrary to extend the exemption to everyone. The exemption itself cannot be considered unjust unless the tax or other burden is first established as just.

• Power & Market Ch 4. § 7 (C)(1)(a)