They didn't have much choice in the matter- the Americans refused to honour the treaties they made with Britain prior to the Revolution, so their only chance to keep their land in Canada was to side with the British. Their support was critical to keeping the Americans out of Upper Canada.
RE: First Peoples
“They didn’t have much choice in the matter- the Americans refused to honour the treaties they made with Britain prior to the Revolution,”
And the Americans refused to honor every treaty with the natives for the next 70 years.