The girl with 50 gp is a bad example because that would essentially be a selective head tax, which are generally frowned upon by society at large. Taxing some gold while she engages in state-protected commerce is a whole different story; she never agreed to the tax, but her purchases are partially enabled by the people around her and she was aware of local taxes when she decided to spend her gold there. So I'd consider the arbitrary seizure of her gold to be far worse than most real-life taxes.
I can give 50 gp to my neighbor right now. You taxing me didn't enable me to do so. That's a non sequitur. If I want to acquire personal security or voluntarily pay into a system that protects my rights, I don't have a problem with that.
Taxes are not voluntary. And even if I were aware that taxes in my locale existed, still doesn't justify its application. I've had robbers in my neighborhood too, but that doesn't justify theft does it?