Only because such cases have not made it to the courts.
I don't expect the government to subsidize me, but I don't want them to take my money and offer me a "service" I don't want. As long as they insist on doing the latter, they can at least return my money if I choose another option.
Should the person who objects to the war in Iraq be able to say that they don't want the government to take their money for a cause they don't want and don't support? Should I be able to say that I don't believe it is the government's business rebuilding Louisiana, Mississippi, and Texas after the hurricanes and refuse to pay a portion of my taxes as a result. People would look at me funny if I did and I have no doubt I would receive a visit from the IRS asking uncomfortable questions.
Any voucher that exceeds the portion of the taxes you pay that go to support public primary and secondary schools is a subsidy. It is the government giving you something for nothing, and providing you a service just because you don't want to pay for it. As conservatives we should be opposed to government hand-outs and not finding new and useful ways of obtaining more. If you don't think the government should be in public schools in the first place, we might find areas of agreement in that arguement. But if you think the government should be in the business of subsidizing private schools then we part ways.
I would be content with a full school tax refund, but that is certainly unrealistic. Further, keep in mind that we pay school taxes our entire lives; education only lasts 12 years, so even many people collecting vouchers worth more than they pay in a given year end up paying more to the government over their lifetimes.
Here, the issue isn't really whether the government is subsidizing education--that's not going to change--it's how the government is doing it. Right now its fully subsidizing one education option--public schools--essentially giving parents who choose that option $8,000 (or whatever the per student cost is) worth of education free, while giving nothing to parents who make other choices. Why would it be any worse to give that $8,000 to all children, and let their families decide how best to spend it? There would certainly be advantages to this choice.
If you don't think the government should be in public schools in the first place, we might find areas of agreement in that arguement.
I would fully support getting rid of public schools. Would you propose the government leaving education entirely, or still funding it somehow? If so, how? I'm certainly open to ideas, since there are very few I've seen that actually seem good. The current one is just really bad.