Why do Libertarians say "Taxation is Theft"?

A reddit user asked a question tonight about why "taxation is theft", whether there's more behind the phrase, and how governments could be expected to function without taxes. I'm rather pleased with the answer I gave:

I'll answer with a couple of quotes from Lysander Spooner, one of the philosophical giants that Libertarians love:

"If taxation without consent is not robbery, then any band of robbers have only to declare themselves a government, and all their robberies are legalized."

And:

"Taxation without consent is as plainly robbery, when enforced against one man, as when enforced against millions; and it is not to be imagined that juries could be blind to so self-evident a principle. Taking a man’s money without his consent, is also as much robbery, when it is done by millions of men, acting in concert, and calling themselves a government, as when it is done by a single individual, acting on his own responsibility, and calling himself a highwayman. Neither the numbers engaged in the act, nor the different characters they assume as a cover for the act, alter the nature of the act itself."

The cornerstone of Libertarian philosophy is the idea that voluntary consent is necessary for any interaction to be considered moral. Taxes aren't voluntary, no matter what the IRS says: Fail to pay them, and you will be punished.

As to how a government can be expected to function without taxes, that's a question we are asked very often. And it has been addressed in both theoretical and practical ways by people who study this far more than I have. But, I'll point this out: Many of the functions where people take taxes for granted, there's usually some place out there that handles at least that one function in another way. It might be an HOA taking care of it's own roads, or waterways, or fire protection. Or it might be funding with reasonable fees attached to services. Thinking that taxes are the only way to solve a problem is lazy.