But a tax is the people (through their government) setting their price to allow the negative externality. You pay the tax, and you can pollute as much as you want to (until we raise the taxes again).
Of course taxes can be badly designed, but he basic idea is sound.
A tax is a market mechanism, with the people (the government) being the only "seller" of "pollution credits".
Only people who have a very limited (and sometimes deliberately so) understanding of markets deny that taxes are sometimes an essential market correction.