Why was it unfair to tax the colonies?

Answer 1

At the time of the American Revolution, the complaint was not taxation per se, it was about taxation without representation. The American colonists paid taxes to the British government but had no say in that government, and no representative in the British parliament, who might be able to influence the way taxes were collected and spent. That is why it was unfair. Taxation itself is accepted, in that it is understood that people derive services from a government, and that those services have to be paid for by taxes. But people also want to be able to decide what services they want and how much they are prepared to pay for them.

Answer 2

It would have been fair because as you state they were the "colonies", i.e., still British territory, before gaining independence as the USA.