"The idea that the state should subsidise giving to good causes..."
Their argument starts with the idea that letting people keep their own money is a subsidy, and that giving to charity is a form of personal consumption.
Those are non-starting opinions for lots of people, including me. The government works for the people and taxes are the only equitable way for it to generate income. And giving away to charity is the exact opposite of personal consumption, since a person is helping society, not themselves, in ways they find most meaningful.
Ugh, just turns my stomach "gives the wealthiest taxpayers a disproportionate role in allocating public resources." A person's income is not a public resource, it's their income, which we vote to tax a portion of.
I think it's reasonable to say that it's a subsidy to tax everything except one thing, since it makes investment in that one thing relatively more attractive than it otherwise would be.
> giving away to charity is the exact opposite of personal consumption, since a person is helping society, not themselves, in ways they find most meaningful.
A charitable foundation can be just about anything. Many wealthy people have their own that almost entirely further their own interests (e.g. an "art museum" next door to them, open only by appointment with them). Or a political advocacy group can be structured as a charity. Or something indistinguishable from an ordinary business (e.g. IKEA). It's very possible for charities to act against the interests of society at large.
If you really want your money to help society, tax is the best way for it to do so. I can maybe see an argument for a narrow class of charities being tax-exempt, but not charitable donations in general under our current definitions.
> And giving away to charity is the exact opposite of personal consumption, since a person is helping society, not themselves, in ways they find most meaningful.
But see, for a counter example, Eton which is an exclusive fee-paying school and also a charity.
I think part of the justification is that people who donate sometimes donate to charities that they own or control and promptly spend the money on themselves to avoid taxes. This isn't even mostly true but there are cases where this happens.