Henry never actually calls George III by name in this speech.

Instead, he refers to "the throne," saying,

We have petitioned; we have remonstrated; we have supplicated; we have prostrated ourselves before the throne, and have implored it's interposition to arrest the tyrannical hands of the ministry and of Parliament. (40)

Remember that independence hasn't yet been declared. Even Henry is thinking of this as a civil war as opposed to a revolution. He doesn't want to offend the king too much when he's thinking eventually they'll make friends again.

We think we'll adopt this custom to deal with people who annoy us. We'll just call them by the furniture they're sitting in until they come around.

Yeah, that'll show the Ergonomic Desk Chair.