This isn't a pun. We swear.
Okay, it's kind of a pun—it's just a lame one. The idea of a "duty" as something you pay is related to the idea of "duty" as something you're compelled to do. Either way, you have to do something you might choose otherwise not to do.
For the British crown it was a literal duty for the colonists—pay this for these goods—as well as the figurative duty of loyal subjects. The logic of the Stamp Act was this: the colonists were benefitting from being British subjects, so they should have to pay for the privilege.
Because a legitimate government is a creation of the people, any action it takes must be supported by the people.
The Stamp Act was merely one half of a transaction between government and citizen. The government provided a service, and when presented with the bill, the citizenry revolted.
The Stamp Act was a new rule and it was attempting to pay for order. During the Seven Years War, the colonies were in chaos. The French were attacking from their strongholds in New France (modern-day Quebec and the Ohio River Valley), and had more Native American allies than anyone was comfortable with.
In the wake of the war, order had to be re-established. The idea would be that a permanent force of redcoats would head off future hostilities before they got out of hand. The problem was: England had already spent that money to win the war. Hence, the Stamp Act. Those British really needed cash, ASAP.
The Stamp Act was critical to fund Britain's foothold on the Americas. Without a standing army, they were in danger of being kicked off as soon as the Native Americans put aside their differences and united.
The Stamp Act was less about money and more about reminding the colonies who was in charge.
The Stamp Act itself isn't about the duality of freedom and tyranny. When Parliament wrote it, they weren't even thinking about this theme at all. As soon as the decree hit American shores though, that's all it was about.
It's like the act was more important for how it was interpreted than how it was intended. (That's true for a lot of things, incidentally.) So just because Parliament never intended it to be a referendum on freedom versus tyranny, that didn't matter.
Good news, though: America got a country out of it.
In levying a tax on the colonies without allowing them to vote on it, the British crown had gone from legitimate rule to tyranny.
The colonies were happy to reap the benefits of British rule, but as soon as the bill came due, they called it, unjustly, tyranny.
The Stamp Act doesn't have much to say on the subject of legitimacy. Everyone around it has a ton to say about the matter, though.
For British Parliament, the legitimacy of both their rule and the Stamp Act was assumed. Of course they got to make laws. That's how British government has worked and will keep on working. It wasn't even a question.
For the Patriots, the idea of the Stamp Act cut right to the point of legitimacy. They believed that because they weren't represented in Parliament, the crown had no right to levy any kind of taxes against them. They had some legal standing for it, too. The Stamp Act ended up being what happens when one side assumes it has the right to do something, and the other vehemently denies it.
At the time of the Stamp Act, the British crown was not a legitimate government for the colonies, and this legislation made revolution a moral imperative.
At the time of the Stamp Act, the British crown was a legitimate government for the colonies, and was merely asking to be paid for services already rendered.