Why were the American colonists angry about the taxes imposed by British government on the colonies

Because the taxes didn't come with appropriate representation in the British Parliament.

American colonists were angry about the taxes imposed on them by the British government on their colonies, as the saying goes, "No taxation without representation".

The colonists had no form of representation at that time, so they had no say about the taxes that they were forced to pay. This led to the infamous Boston Tea Party.