If the colonists paid the tax without protest, it would have established a precedent. It would establish the right of Parliament to tax them without their consent. Throughout the colonies people protested, and refused to let the tea be unloaded. The British Governor refused to let the ships leave Boston Harbor. These men established their own precedent.
Theft is too mild a word.
Typically, a thief strikes only once, and doesn't pretend that his robbery is legitimate. Taxation is actually slavery. The medieval serf was required to work his master's land one day a week. Today's tax slaves are required to work more than two days a week to pay taxes. Well, I have not given my consent.