The Boston Tea Party was a mostly symbolic revolt against "taxation without representation," and justified by the notion that unelected officials, especially across the ocean, should not have the power to levy taxes at will. In Johnny Tremain, it is the reaction to the Tea Party that plants seeds of revolution:
There had been many a moderate man who had thought the Tea Party a bit lawless and was now ready to vote payment for the tea. But when these men heard how cruelly the Town was to be punished, they swore it would never be paid for. And those other thirteen colonies. Up to this time many of them had had little interest in Boston's struggles. Now they were united as never before. The punishment united the often jealous, often indifferent, separate colonies, as the Tea Party itself had not.
(Forbes, Johnny Tremain, Google Books)
Boston's reaction to the tax -- initially low enough to be ignored -- was one of independence from the British monarchy. By destroying the tea instead of stealing it, they showed that they were not willing to submit to British rule any longer; in this sense, Boston was the head of the fledgling United States as it fought against its country of origin. Their actions were justified by the needs of the day and by the morality of freedom, not by legal means, and so could be seen as revolution in its purest form.