You are falling into a predictable fallacy. Predictable in that this comes up all the time when people talk about the government. The fact that the government provides a service does not imply that the service would not exist if the government did not do it. We could absolutely have cabs without the TB problem without the government. In fact, in areas where the government does not enforce its own monopoly, you see competition with government-provided services flourishing. UPS, FedEX, and DHL versus USPS. Private college loan providers. Private universities. Private schools. On and on. The idea that without the government doing it, we are going to have rampant TB-infested taxi drivers is just silly. No one wants to use such a taxi and they do not need the government to tell them that.
> "This is the big problem I have with the libertarian free market ideal: it's fundamentally reactive, not proactive."
I do not know if I should take that statement seriously. I expect if you think over that statement for even 30 seconds you can see why it is both a complete non sequitur and insane.
1) Laws are obviously reactive. I would bet all the money in my pockets against all the money in your pockets that TB testing was put in effect as a reaction to an event in which a taxi, bus, or train driver was spreading TB.
2) The free market is exactly as reactive or proactive as people are. This is its virtue. People providing other people what they want is the whole point, both of markets and, it is claimed by those who believe in the State, the government. No organization, government or otherwise, can get away from being reactive to human needs unless it provides things people do not want or need, and in what way would so doing be good?
3) It does not matter anyway. The point of all this is that people should be free to pursue their desires, regardless of the considerations of people who think they know better and are worried about things being "reactive" or "proactive". These concerns are simply petty against the moral position of freedom.
Look, I enjoy a good discussion on the internet as much as anyone. I have learned a lot from reading comments on HN. But I get the impression that you have not read any of the good arguments against your position. I am a poor substitute for any of the major thinkers in the libertarian tradition. If you want to save yourself reading, you can even watch videos of Ayn Rand, Milton Friedman, or Thomas Sowell on YouTube. I recommend Stefan Molyneux's show, Free Domain Radio. He discusses these issues often. You could even call into his show and have a long discussion with him on the topic. He entertains such calls often.