Why do libertarians support school vouchers?

Gary Johnson is running for the Republican nomination for president. Johnson was the governor of New Mexico for 8 years around the turn of the century. He is considered one of the purest libertarian politicians. He favor the legalization of drugs, for example. One of his "signature" issues is school vouchers. He worked hard for them in New Mexico when he was governor. The way school vouchers work is that the government gives kids (or their families) vouchers with which to pay tuition at the private school of their choice. It's understandable that a libertarian would favor private schools over public schools. What I don't understand is how libertarians justify having the government pay for those private schools. Why isn't the libertarian position that if someone wants education—for himself or his children—it is up to that person to find a way to get it. That seems more consistent with libertarian principles.

I left a note on Johnson's web site asking that question. So far, no reply.

The larger question is what libertarians think the role of government should be. Should the government run a fire department? If not, must we have private fire departments that protect us. What if a house whose owner does not buy protection from a private fire department catches fire—and endangers the houses near it. Do the neighbors have the right to ask their private fire departments to spray water on the burning house? Doesn't that violate the property rights of the person who owns the burning house? What if he wants it to burn? Presumably he would be liable for damage the fire caused to the neighbors' houses. But what if he doesn't have the money to pay for those damages? Must we then buy insurance to protect ourselves from "uninsured homeowners?" I can just imagine the legal battles that will generate. We will need 10 times as many lawyers as we have now. And we will all be far worse off.

So what should the role of the government be according to libertarians?